One possible reason would be to "group" reports: if 95% (or even 100%) of a certain error come from a single computer then there's either a very obscure bug or something wrong with that computer. If all reports come from different computers then it's a much more serious issue.

IP addresses aren't really super-reliable for this (corporate networks, ISP NAT), but many people still are under the misapprehension that they are, and it's probably reliable enough for this purpose.

I don't know if that's the reason; but it could be.