Wiring kits?

I'm having a hard time understanding why people would pay so much money for a bunch of wires with a fuse box? I mean they will be putting a fuse box in there where the old one went? Then they'll have to run each wire to where it goes? Even if it is color coded you'll have to really travel that wire all the way to where they go so you can hook it up? At what point do you not just buy a $15 spool of wire and do this yourself? All I really see is someone taking a factory harness out and copying all the wires and lengths and colors that were ran? Really all I see is a $10 fuse box in a $15 roll of wire. I see these wiring kits and they're well over $500 and I always assume that includes installation easily!