I am playing a Shadowcaster in a 3.5e Forgotten Realms campaign and I took Still Mystery at 1st level, which lets me cast mysteries without the use of somatic components. What is the benefit of being able to do that?
You cannot perform somatic components if you are restrained, if your hands are full, or if your armor gets in the way. Without somatic components, you can keep your hands full and ignore any arcane spell failure on your armor, and restraining you will not prevent your magic. Plus, you don’t have to move your arms or hands around to cast your mysteries—even in situations where you can it might be better if you don’t have to, to make it less obvious what you’re doing. Still Mystery is a pretty good feat for a shadowcaster.
You should know, though, that shadowcaster is a kind of mediocre class. It’s certainly not unplayable, but particularly at low levels you will be routinely reduced to plinking away with your crossbow since you’ve used up all your mysteries (or you’re trying to conserve the very few you have). And despite having far less stamina than a wizard or sorcerer, mysteries are also quite a bit less powerful than arcane spells. So all around, it’s not great. Ari Marmell, the class’s author, did write up some suggested improvements to the class, which help, but even with them the class is still a bit lackluster—you get a reasonable number of mysteries per day, so there’s less risk of turning into a poor crossbowman, but it does nothing for the fact that the mysteries themselves aren’t so great.