I'm still relatively new myself, but I like wild colors and fabric patterns so I use solids to give the eye a place to "rest" after taking in all the wildness. I was doing a split rail quilt in my "learn to quilt" class and, since I was mixing the fabrics together from 2 precut jelly roll packs, the patterns would have been way too busy and chaotic if I'd put the blocks directly against one another. Would have ruined the entire effort in my eyes (and I could tell my quilting teacher was trying to figure out how to mention that might be an issue as soon as she saw my fabrics... bless her heart lol) Instead, I threw sashing with a solid color in between the blocks and that worked out great. So sashing and backgrounds are good places to use solids in my eyes, and I'm partial to at least one solid or "read as solid" border (usually inner border) per quilt. It helps to keep a crisp and clear distinction between the various areas/spaces of the quilt design.
Also, sometimes if I'm working with a lot of fabric patterns, I'll throw in solids as "filler" (because the solids I use are less expensive than patterned fabric) If I do that, I try to use at least 2 so that it doesn't look weird to have all my fabrics be patterned except for one. That's purely to save some dough.