The idea here is to be lighter-weight than profiles, or the similar feature in Chrome. I've got three different containers going right now, side-by-side in one browser window. In addition to the per-tab basis, providing separation within a profile also means you don't have to set your preferences, addons, etc. independently each time.

For example, one of the major use cases for this sort of separation is being able to use multiple accounts on a site like Twitter or Facebook (which don't support multi-account) or Gmail (which does, but it's a bit rough). There's no reason I should have to have separate windows or separate addons for each of those accounts.