What does the dealer do to same mower that box store don't,[new,one,setting it up I mean],maybe grease it,make sure deck is correct as far as level,etc,correct tire pressure,not much else wouldn't think,and anyone even buying from dealer should check all that out before using.
Now buying from dealer gives you a place to bring it to when it breaks,but there are small engine repair places everywhere[and they are small and local too].
Guess if you couldn't sharpen your own blades and change the oil,you had better buy from a dealer,but as far as I'm concerned if you are buying a cheap lawnmower,the word there is cheap,you are after a cheap lawnmower,so go get the cheapest one you can find,and those big box store prices are hard to beat.
I bought a snapper off a dealer by the way,cause box stores don't sell this model snapper!