Why write $P(x)dx+Q(y)dy= 0$ instead of $P(x)+Q(y)y'=0$?

Question

I see ODEs like $P(x)dx+Q(y)dy= 0$ in the context of exact differential equations.

My understanding is that this is another expression of $P(x)+Q(y)y'=0$.

Why write $P(x)dx+Q(y)dy= 0$ instead of $P(x)+Q(y)y'=0$?