I've always presumed that, besides wanting good guality fabric that many quilters are more comfortable making a quilt using fabrics from a "line" or a "set" or whatever its called. It gives confidence that the colors go together well.
Then there are others who relish the act of moving fabrics around their quilt table for a while, or a day or two, before they begin a new quilt.
A few years ago I walked into a quilt store and on one gondola were the same colors (same company) used in several different ways (leaves, dots, stripes, etc.) in colors I love. I bought about 5 variations because I wanted to make pillow covers for the living room. A few weeks after I was proudly done I realized it makes my living room bland. I miss the assorted hodge-podge and find the coordinated pillows dull, too bland, predictable. Oh well, when I finish the next 30 quilts that are bouncing around in my head I may redo the pillows. :) I should add that all the colors were slightly muted and I really like bright colors.