Enlighten me

So my boss just changed a policy about using sick/vacation time. We can only use it on days that we are out not if we go home early/come in late. Ok fine like whatever. But it’s the sick/vacation time I’VE accrued, so why am I not allowed to use it as I please? What if money is so tight but I get the flu and leave 15min after my shift started before the weekend? Like wtf is the difference if I use it now or later? The money is still gonna be added to my check at some point so...? It’s the time I earned.

Maybe I’m ignorant about it, so someone tell me one reason that this even makes sense.