Usually implies “I’m” fixing to. Often said without much emphasis, as it’s just introducing the important part of the phrase. I think it’s actually a pretty neat way to keep the emphasis where it needs to be.
“Finna get outta here” uses 3/4 of the phrase to convey the important action of “leaving”
vs. “I’m fixing to get out of here” uses 1/2 of the phrase on useless info that “I” am the one doing the leaving and that it hasn’t happened yet but is about to.