What is the purpose of the human resistance fighters I meet during the missions?
The human resistance fighters will attack Terminators and other Machines in order to help clear your way through a mission. But you should also try to protect them : the more humans that survive, the more points you get.
What is the use of the crawling type of movement?
More powerful weapons such as the Plasma Cannon come with very limited ammunition. You should save such weapons for the tougher enemies and to use the more common weapons whenever possible.