I'm looking to buy new valve covers for my 1968 427BB and they come tall or standard, my question is: what purpose do the tall valve covers have? or are they just for looks?

The big block Corvettes generally take the short valve covers for clearance. The C2 body Vette with big blocks use a specific left side valve cover with a large indent to clear the power brake booster and master cylinder assembly.

The tall valve covers could fit in other cars in the line as well as trucks, though I believe the short style was standard. If tall ones will fit without clearance problems in your car, go for if you like. If you ever install roller rockers, you'll definitely need the tall ones.Gunslinger38484.7444444444

I do have roller rockers installed so I guess I need the tall ones.