About as many reasons as there are people who own Teslas.
Seriously, though...
Opportunity: Go out for a bite to eat/Christmas shopping after work and get some free juice. This includes the lowest of the lows who plug in and then leave with a friend to do other things for a couple of hours.
Speed: You arrive home with 35%, and need to leave in two hours for a commitment.
Doing stuff: You need to organize your papers for your tax return
, so you grab your information and organize while charging for free.
No home hook up: People who rent or live in condos/townhouses that cannot charge at home without hefty fees assessed.
Parsimony: Some people are inherently cheap, frugal, or thrifty, and money saved is more important than their time.
Justification: There was a time when Tesla charged $2,000 for the Model S60 to enable unlimited Supercharging (unlike the S85 where it was standard.) Some folks got it in their heads that they "paid" for Supercharging whether with a 60 or an 85, and therefore they feel justified in plugging in.
Because I can: Some people feel entitled "because I can," and will do it regardless.