You build a room within a room when you do not want to utilize the entire room so you can optimize your environment, as Benny said. Otherwise, you waste light and are cooling space that does not need to be cooled, etc, etc. Building a room within a room will not, however, help with preventing spider mites. Mites are generally brought in on your clothing or body from outside or by pets. Cleanliness of your grow room will not prevent/deter mites, although it will prevent other problems.
As a plant gets close to harvest, the calyx can swell and look like seeds. This freaks me out a lot. Sometimes I could just swear that there are seeds in there...