[yangmeyer]

The best thing about this "assign after payment in full" clause: It makes potential disputes about unpaid invoices a copyright infringement issue. And crucially, legal costs for copyright cases in the US are paid by the infringing party.

(IANA lawyer, but I learned this from one while contracting.)

[Argorak]

The most important part is that it avoids many cases where the client tries to "ship off" the software. (e.g. by selling it to another party for a penny and going bankrupt itself)

This is in some cases fraudulent in itself, but being able to block the sale is a much clearer case.