We started with no organization. This is fine on small sites, but if you're coding something larger than 3 pages it gets hairy quickly.
Then OOCSS came out. OOCSS says, "make things out of small building blocks and then put them together to make bigger things - like LEGOs!"
https://www.smashingmagazine.com/2011/12/an-introduction-to-object-oriented-css-oocss/