Without pretreatment, DTG white ink soaks into the fibers and comes out thin and blotchy. The pretreat holds the ink at the surface as it lands, so the white layer builds opacity and the colors printed on it stay bright. Printer makers specify their own solutions, and some are mixed stronger for dark garments and weaker for light ones.
Application has to be even. Too little and the white looks weak or washes out; too much and the fabric can feel stiff or show a visible rectangle, stain or yellowing around the print, especially on light colors and some heathers. These marks often fade after a wash, but it is worth asking. Polyester garments may need a different pretreatment altogether.
If you are ordering DTG on dark or colored blanks, ask whether pretreat marks are washed out before delivery and how the shop tests an untried blank. DTF skips this step entirely. See types of shirt printing.