The current era cars use ground effect to produce most of their downforce. This air that flows through the underbody is then thrown almost vertically up in the air when it leaves the diffuser. The idea behind this rules framework is that this will get the dirty air out of the way (by tossing it way above the cars) so that following closely is easier. Previous-era cars left a huge wake of dirty air behind them which made following for extended periods of time impossible.

The side effect of throwing the dirty air high up after it leaves the diffuser is that the same thing happens with water when the track is wet, since it also gets sucked into the underbody. That's why you see these cars produce "rooster tail" spray when running in the wet. This type of spray hampers visibility enormously and is currently the biggest obstacle in the way of proper wet running.

2026 will bring the next major change in the rules framework, so there is a chance cars designed under those rules won't produce the same sort of spray, if they decide to for example move away from ground effect.