As below Also, this always confuses me, "aero doesn't work until you're going over 80mph" it's said a lot, if you go for a cycle with lycra on, you're going to get a higher top speed than if you go for a cycle with a knee length coat on. The coat catching the air is going to slow you down because it's causing drag. Drag and downforce are related and an increase of one will increase the other... to a certain extent.
A wing on a drift car will be at a massively steep angle of attack, basically creating some downforce giving you grip off the line. When fully sideways, yes you'll be relying purely on mechanical grip, but you're rarely 90 degrees sideways unless something has gone wrong. Most of the time you'll be at around 45 degrees or less, meaning that you'll still feel some benefit.
Also the likes of Rhys millen used them as a safe barrier, Its more surface area on the side pushing against the wind to stop it spinning full rotation He even added on one of his cars a full on perspex sheets from the wing to car
For rookies like me, it's all about mechanical grip and a big wing is just because they look cool