I utilize it when I have short term fiat bills I know are coming up. Or if I have a big expenditure in 2 weeks or whatever. Ill sit my fiat there to earn some bitcoin while I wait. I dont want to risk a change in value by having it in bitcoin short term like that. While yes bitcoin price could rip, its not worth the risk of the other side ifnprice falls and I have to sell bitcoin at a lower price just to cover my bills I had already covered 2 weeks before.

Oh I may have misread your question. Cash reserved is in fiat because its cash sat on a limit order or whateever, waiting to execute.

Ah gotcha, I just yeet it whenever I get it. I either can't understand or don't have the patience for anything else.

Because not everyone chooses to be 100% in bitcoin. Some people want to hold some cash reserves for personal reasons. Others are waiting to buy the dip, and earn interest while they have a Target Price Order in.