They’re a waste of time. Seriously. The College Board can ask anything. There are no patterns, no logic, no way to guess what’ll show up.
Your main concern needs to be fully mastering the things you can control, and you cannot control prepositions. There are too many, and they’re too random. Sitting for hours trying to memorize lists of them does not constitute an effective use of your time.Making sure that you can recognize dangling modifiers, comma splices, subject-verb disagreements, pronoun disagreements, and problems in parallel structure does.
Besides, there will usually be a grand total of two preposition questions on SAT Writing, and sometimes not even that. Even if you miss them, you can still score an 800. And frankly, as long you’re above 750, no one really cares all that much. On occasion, people even get into top schools with — get ready for this one — scores in the low 700s!
Now, if you have truly and thoroughly mastered every single other grammar rule tested on the SAT — to the point at which you do not ever miss a single non-idiom/preposition question — you can consider looking at some prepositions. But at that point, it’s just not really worth your time to sit there memorizing idiom after idiom when you could be doing things that will actually make you an interesting, appealing candidate to colleges (or simply an interesting person period).
So please, do yourself a favor and go finish your physics homework instead.