From The OAuth / App Anti-Pattern | Terence Eden has a Blog via @schmkr:
Typing your username and password into a third party site is bad idea. A really bad idea. I mean, you may think it’s a bad idea to give your bank details to a Nigerian prince but that’s just peanuts compared to giving away your password to an untrusted site! So, that’s why we use OAuth. Rather than handing details to a random site, we authenticate against a trusted site which then redirects us back with an authentication token. That’s all well and good on the web, but on mobile apps it becomes a little more difficult (ed. hint: you seem to get redirected to a genuine webpage, but can you be sure about that? - murb).</blockquote>