These guys get a chance they wouldn't otherwise have to show off their football skills as well as other subjective skills like effort, smarts, leadership etc.
The risk of "bad" critique (as well as injury) is low because the alternative is just a player's film or what other people say about the player versus getting a chance to make a 1st impression in a football setting. The upside potential is more valuable than the downside risk, i.e., Williams isn't going to get downgraded because he only averaged 3.5 yards per carry versus making a very positive impression in practice. Most of the coaches don't even stay for the game.
The game itself is pretty pointless(the tension than your typical all-star game) except for Reese's who gets 2 plus hours of pure promotion. Games rules are pretty limiting, .i.e no blitzing, only 3 or 4 secondary coverage schemes are allowed, limited offensive packages, no shifting/motion etc, so you don't get real game conditions.
Except for the qbs, fewer than 10 of these guys will go in the 1st two days. Lots of unheralded players, smaller schools, lower exposure and guys who aren't going to get invited to the combine. And they do get to show off their football skills in practice setting. which works for guys like Tranquil, Coney and Williams who won't wow at combine type skills but are good football players.