My attempt at this puzzle is:
Updated after OP improvements - not limited to vehicle gears due to later clues
Sometimes I am automatically forgotten
A vehicle may have automatic gears so the driver can forget which gear they are in.
I always have a purpose
Gear is equipment that is used for a particular purpose.
I can help out in a hostile situation
Riot gear in a hostile situation (OP's help)
Original - Adverse weather conditions call for appropriate clothing (gear). Also can be weapons.
Firstly, I'll get you nowhere fast
First gear in a car won’t get you anywhere fast
On the flip side, I can even take you backwards.
In reverse gear you go backwards