Good questions.  Yes, anywhere Fraxel is used it builds collagen but the real issue is  -  does it make a visible difference that we all can see, rather than a microscopic difference.  Stretch marks do respond to Fraxel but the studies on this are small and hard to interpret.  I think it's fair to say that expecting a 30-70% improvement is reasonable but it takes a LOT of treatments (think 5-10).  That makes the whole process a bit expensive but it may be worth it to some. 

There are excellent studies showing improvement in the appearance of sun damage on the arms, legs, chest, necks and faces, of course.   Generally for the chest up expect 3-6 treatments needed depending on the severity of the sun damage.  Arms and legs need more treatments and take longer to heal.  There are nice studies showing improvement in precancerous lesions (actinic keratoses) as well.   Dr I