Once a very popular form of filtration, we now find them to be completely obsolete. New technology especially from canister filters, can circulate and aerate water via long probes that insert into the water to offer the same level of circulation that under gravel filters have. Another think we don’t like is over time, detritus can sink through the plate and prove difficult to siphon out. An easy ammonia spike and algae bloom is the result of trapped detritus and food particles. If someone tells you under gravel filters are good for aeration, we would counter argue that an air bubble wall powered by an air pump provides the same level of aeration.
A reason why some people still use under gravel filters is to provide a layer of protection between the bottom glass and any dropped ornaments. If you have young children with their hands in the aquarium helping you with maintenance, a dropped rock is not going to cause a crack in the glass since the layer of plastic will adsorb the impact.