The difference between the two is that work for a client can typically only be sold once. Work like creating an icon set or writing a book can be sold infinitely and require virtually no post-support. Additionally, those streams of income can continue while you're working on more client work if that's your thing.
Not necessarily, for some client work I've reduced my price in exchange for full ownership (or rather, co-ownership) of the product I've developed. That way I can do with it what I want.
Except you can also scale your consulting business by hiring or tying up with other devs. This means you can still earn while others are working for you!
