1. Americans are too damn lazy to say a couple frickin words so they say 'ima' intead of 'i am going to/I am a' because it's only one word, as aposed to 3 or 4 words.
2. 'Ima' is a reallyyy fun word to say!!! Soooo much more fun to say than 'I am going to/I am a'!!!!!
Asian Guy: I am moving back to China.
2. Random Person: Yo, i'm gonna go hit up the movies. Who's comin'?
Bored Person: I am going to come.
Happy and Fun Person: Ima come!
Notice the use of three distinct elisions to turn a four word phrase into a three letter word: "I am going to" > "I'm going to" > "I'm gonna" > "ima."