Take a look at the following which discusses what backups we do on your site
In answer to your other questions
We would provide your backup as soon as reasonably possible. For example, if this was requested on a Friday night, its likely it would be Monday morning. If you requested it midday on a Wednesday, its very likely someone will provide it pretty quickly (for example if you requested now, I would probably see it shortly and get it done)
People request backups for various reasons. Some just like to have a copy for their own peace of mind, some because they use the database for stats and more. There isnt any real reason for you to hold a copy yourself unless you really want to, but of course the data is yours, so if you want a copy you just need to ask and we can sort it for you 🙂