How do you raise it and what is so important about it.

Talent purchases in the character trees, and it allows you to roll another Force die with each rank. With more dice comes more total Force pips generated which in turn allows more effects, or increases effects of Force powers. Some Force powers also require a rating of 2 or even 3 to even be able to start purchasing them. Force ratings represent the total number of Force dice you have access to, and in some powers and Talents you can commit them to ongoing beneficial effects. That's the long answer. Short answer, cuz they're good and make you awesome.