Combined with webmail, this can be used to transfer files into or out of situations when you have nothing but internet access, and don't want to use a third-party service. It's also useful for keeping certain kinds of important information, which is then easily accessible anywhere via webmail.
By keeping the items in draft form, and not actually sending them to myself, I can edit them as much as I want, replacing their content and attachments, without any clumsy re-send workflow.
I'm already logging into e-mail, so using that is simpler than using e-mail, plus some other, separate, online service. On-line storage services have complicated UI, that support sharing workflows and whatnot that are completely irrelevant to this use case.
I run the mail server myself, so the data "stays at home". It's on RAID and backed up more regularly than some of my other storage locations, which is a good reason to park certain information there.
I could use an online service for it but most of the time I find these sites while at work and online storage sites are usually blocked for us. But email is open and allows me to send data to my personal account to be explored later when I have time.
Whenever I find anything that I need to read later, I email email myself, so that in the weekend hopefully I can catch them up.
So, basically, email (and also Calendar) is my todo list.