Why decay, as a mechanic?

A lot of abilities have decays, e.g. Slow x%, decay over y seconds.

Ok so let's say you have a slow of 10%, decay linearly over 1 second. That's the same as 5% slow for 1 second in terms of movement, but it is stickier immediately after the slow for stuff like targetting skillshots, etc. I feel like this might help newer players who are less adept with skillshots vs the flat slow. I feel like it might just make all champions a little more reliable.

Is there anything I'm missing here? It's an interesting idea, from a design perspective.