In League of Legends, some players make "smurf" accounts, which are typically lower-levelled or lower-ranked accounts to play on instead of a main account. Is there a good reason for creating a smurf account in DotA 2?
If so, can I have two DotA 2 accounts on one Steam account, or do I have to use a separate Steam account?
You can make a smurf account to relive the MMR determination phase of a player. Afterwards, you'd probably go back to what you are in your first account. Another reason can be if you don't want your first account identity to be discovered when you want to play some particular play that are typically frowned upon.
You can only have 1 DOTA2 account per Steam account as it is tied to each other. You can make another Steam account, however, to make a smurf account for DOTA2.