2278.214692 megabytes (and counting) of free storage is not an amount to be sniffed at especially when it’s on servers that you’d expect to be well maintained.
GMail Drive shell extension is an interesting implementation of a Gmail-driven remote storage facility to allow you to map your Gmail account to your Windows file system as a drive. How long before Google decide to take action to prevent their email service being used in this manner is anyones guess.
Another, less illegitimate(?) use of a Gmail account is as storage for remote backups.