To hide his true identity, duh Lol
Deceitfulness, and like another member has stated; to hide his true identity from the Akatsuki. A goofy demeanor wouldn't set off any alarms within such an organization. However, someone with clear malicious intent rooted in their voice and actions would, and people would have likely conspired against him while Nagato was alive.
Why did he even have to get involved with Akatsuki though?
Why did he even have to get involved with Akatsuki though?
Manipulating a large amount of powerful missing ninja so he could gain all the Bijuu's with less effort and much better efficiency (Many Ninja with unique powers > Obito himself getting the Bijuu's).
They had already collected just about all the Bijuus before Tobi even got involved with Akatsuki.
Tobi did nothing in their ranks. I fail to see why he teamed up with Deidara, he didn't benefit from it and easily could have just stayed on the sidelines watching.