The mats are to protect your cutting strip (dont know why they call it a cutting strip, it should be called the strip that the blade operates over) your cutting strip should remain unblemished if you use the correct depths for the material you are using, i.e. only cut through the vinyl, not the backing paper as well.
Of course when you are cutting decals from a long length of vinyl you are unable to use a mat, but at all other times it's best to use it as like Edward says, it stops the vinyl from possibly moving askew. and it's a must for getting fine detail cuts.
mats should last a long time, the glue is the first to go but can be replaced by cleaning the old glue off, which is a double sided tape basically, and giving it a quick spray of 3m remount spray