Something about Christianity, or at least the way most Christians talk about Christianity, confuses me

Most Christians will talk about "Gods Plan". Which makes sense to me, the Christian God is omniscient and omnipresent so he could have a super well laid out plan for every micro second of human history.

But like, doesn't this kind of defeat the purpose of prayer? Like if a family member gets sick, what's the point to praying to God about it. Whatever happens is part of his "plan", so there really isn't any chance you're going to change his mind on whether Grannie is going to pass.

Same with things "going against Gods Plan". Gods plan should have every contingency accounted for, so it really shouldn't matter what anyone does. Is there a chance that if too many people are gay that will derail Gods plan and everything will be fucked? Or did Gods Plan account for me being a big gay commie? Is the idea that you can't fuck up Gods master plan, but if you do a bunch of weird crap God doesn't like it will throw things off slightly and God will have to compensate which he finds really annoying?