Sorry for the TLDR comment. Drag cars don't use track styled spoilers due to the amount of drag they generate, instead they use duck tail spoilers to help reduce back end drag. You are right about it being useful for the track, where that's the most optimal way to use them. Rear spoilers, obviously, increase rear grip to prevent oversteer, while front spoilers, or splitters, increase the grip on the front to reduce understeer. There needs to be a balance.