I only buy retail if I really need something or just to try out a product or I can't find it from my sources. Other than that I buy all my chemicals and supplies from a wholesale distributor.
Some stuff bought at retail has it's purposes. E1 tire swipes, "insert company name here" tire foam for the wells, etc. I think you can get some of Megs Pro Line now at some CTs too.
Smaller amounts. (If you like to "just try" stuff, you don't need a gallon, or even 32 oz.)
Handier. You might be shopping and wander on by the automotive section of a department store, whereas you would need to make a specific trip to the dealer.
Sometimes the retail products are more "idiot proof."
And maybe even the retail products might smell nicer/have a fancier packaging.