1. Every other Avenger is getting their own solo movie:
Before Avengers even came out Iron man had two movies and Thor and Captain America also just received their own movie a year before Avenger.The Hulk had two movies with two different actors(Eric Bana and Edward Norton) with Mark Ruffalo's Hulk set to get his own movie and Marvel also announcing a Black Widow solo movie Hawkeye seems to be the only one left out of the action the audience needs a chance to see what the master marksmen is capable of on his own.
2.To see his life outside the Avengers:
In Avengers we don't see much of Hawkeye's life out of SHIELD and the Avengers even if he is an agent every other Avenger was involved in their own work whether it was Tony Stark working in Stark Tower or Captain America at the gym .Hawkeye on the other hand continually just keeps on working as a SHIELD agent they should show who he was before he was a SHIELD agent, an origins story which would be awesome if done right after all he was trained by Trickshot and Swordsman.
3. Redemption for the disappointment with the character so far:
Let's face Jeremy Renner's performance as Hawkeye has so far has been greatly disappointing debuting in Thor with a failed cameo and for the majority of was controlled by Loki so this gives Renner the opportunity to redeem himself by being given the starring role fighting against evil on his own before the Avengers were even created which makes for movie gold again if done right.