What is a derma roller used for?

Derma rolling: The idea behind dermarolling is that small needles puncture the skin thereby enabling better penetration of a serum. The trauma of pricking the skin with needles is also supposed to prompt a repair response leading to improved looking skin. Personally, i regard dermarolling as potentially unhygienic, and unnecessary (as a good serum should be able to penetrate the skin without it being cut.