Maternity underwear?

Samantha • Had our first in November 2015, our second in December 2016, now thanks to and IUD failure we are currently pregnant with our 3rd and couldn't be happier. Maybe I'll finally get a girl.
Ok so one of the many thing my mother in law is driving me nuts about is maternity underwear. What I want to know is what is the point are they more comfortable or are they just a gimmick and another of the many things pregnant women get suckered in to.