The SH Course (or "ASH Program") was originally developed to help ensure that those with the powers of an SH would know how to use them effectively and responsibly.
Since that time, the power and authority of being an SH has been significantly and intentionally reduced so that it largely only affects the duty for which the role was created: hosting stories - or as we typically call them, sessions.
Because sessions involve danger and other potentially life-altering events for player characters, they can have a huge impact on both players and the setting as a whole. For that reason, an SH course is still needed to ensure that anyone with the permissions to run a session is aware of the impacts of those sessions and how to account for them accordingly.
It's also in part because we want to ensure that sessions are enjoyable for a wide variety of players - something that doesn't always happen naturally even with a host of SHs to choose from. The SH course is an attempt at broadening perspectives for new SHs so that they are not only capable of but see the need for more varied and diverse challenges in sessions, and can handle and provide enjoyment for a wide variety of player tastes and character types.
The current course design involves three "shepherded" sessions with an SH mentor and no number-crunching on the part of the ASH, followed by five solo sessions with the ASH effectively serving as a full SH under closer monitoring after the fact.
This provides an easy and accessible point of access to allow a player unsure if they want to be an ASH to try out a few sessions without having to commit to learning the system in depth first. It also provides a form of protection for players in the shepherded sessions and the setting at large against the potentially-problematic impact of a less experienced ASH. The SH mentor can intercept issues for which the ASH has yet to be trained and keep them on course with additional instruction, in case the course itself and the automated testing were not clear enough to cover the necessary situations.
Each stage including the first is preceded by a test meant not just as an assessment of what the ASH knows, but as an instructional tool to inform them of what's expected of an SH in an iterative process. Copious feedback is included so that the tests are automated. These tests are designed as objective assessments - anyone who passes them is considered proficient at that level. Automating the grading and feedback ensures consistent treatment of any ASH, regardless of who they are, so that no one person can choose for themselves who does and doesn't deserve to be an SH.
The final test involves questions with no single right answer that are submitted for review and approval by the SH corps at the time. This is a final assessment of the ASH's ability to perform the duties of an SH, and provides a chance to catch and correct any troubling behaviors or philosophies on the ASH's part before they become a full-fledged SH. Three SHs are needed to approve the ASH so that, again, no one person can decide their readiness either way. Any objections to the ASH proceeding to full-fledged SH must be supported by argument and evidence - there is no option to simply vote 'no' without explaining why.
The moment an ASH becomes an SH, they have equal authority and voice to any other SH in the matters of that position. There is no sense of seniority "elder statesmen" for SHs: anyone who can successfully pass all stages of the course is every bit the SH of any other.
SH Renewal Cycle
The SH course is not just for new ASHs. Even active SHs must renew their credential annually in order to retain their position. The renewal cycle prevents even veteran SHs from veering too far from the established rules. Rules may change over time, and SHs must stay current, which is why the SH renewal tests are also intended as instructional tools to bring them up to speed, if they aren't already.
There are also more subtle changes, notions like "power creep," where an SH can get into bad habits slowly over time without intending or realizing it. The SH renewal process recalibrates the SH's understanding of the current system to ensure that these habits do not endure, so that they don't cause further issues over time.
Character and Item Evaluation
Character and item evaluation duties fell under the SH role due to a combination of overlap in the training and simple necessity.
From an items standpoint, as SHs are typically the only people awarding items, they needed to understand the specifics when creating them so as not to go overboard (or underboard, in rarer cases). Since they needed the knowledge to be an SH anyway, it seemed logical at the time to task them with the responsibility of evaluating the items of others as part of their duties.
Character evaluation falls into a similar realm. SHs create NPCs using the same rules (if not always the same limitations) as characters. SHs also have to learn the Setting before becoming an SH, and must learn how to rule each type of skill. Since that knowledge neatly overlaps with what's required for character evaluation, it's again a case of convenience to task them with character evaluation, rather than developing a redundant role.
The reason only SHs are able to do item and character evaluation is because any errors in item or character evaluation will likely impact SHs most immediately. A player who writes a poorly-written character and gets it approved clearly has an idea in mind to play, and will continue playing the character they wrote until they run into something that encourages them not to - typically an SH or lead. Items that are overpowered force SHs to adjust their numbers disproportionately or completely redesign sessions around unexpected powers.
As a result, SHs have an incentive to police themselves as a group to ensure that evaluation is fair and at least somewhat standardized so that none of them are ever surprised by a mishandled character or item in the middle of a session.