It is optional but if you want to progress bell and Cassie's stories then the desert is a must for clearing. If you're confused about the coordinates and stuff, go to the LD+ wiki for the directions
Its like an extension of the Cassie quest after the don quest, just get to the desert city and explore it thoroughly. You'll see what I mean.
Im not sure what classifies as a "main" quest since the quest system is still vague and it varies on the players interests but the desert quest is a part of the logbook so...