As a new quilter (long time garment sewer), I love precuts. I especially like the YouTube Videos put our by Missouri Star and their Daily Deals
http://www.missouriquiltco.com/shop/dailydeal They tell you how many different prints are in each package and how many (not always which ones) will be repeated.
I made a quick, but pretty quilt for my mil for her birthday using a Jelly Roll.
I made a easy table runner using charms.
Precuts are great for new quilters who do not have a lot of rulers and special tools.
They are also good for people like me who are not the best at coordinating fabrics.
I am currently making a simple dress for my niece using a jelly roll and coordinating yardage. I found the pattern on the Moda Bake Shop.
For well established quilters with lots of rulers, tools, patterns and colour experience they are probably not practical, but they are great for me.