Some babies get themselves in to a routine some don't, but you'll still be hitting quite a few growth spurts in months to come so then it'll all go to pot anyway. Demand feeding is recommended as you are listening to what your baby needs rather than making that decision for them.
For example on a very hot day your baby will be thirsty and may need to feed more regularly for shorter periods - it would be mean to leave them thirsty because it wasn't "time" to feed. Whereas on another day they might want a nice long cuddle and feed for quite a while, or yet another day be too interested in the world to stay on the boob for more than 2mins and be on and off every 5mins till you want to pull your hair out (yes three month olds, I'm talking to you!).
Breastfeeding isn't just about a baby's need for food and drink but also bonding, cuddles, reassurance etc. and demand feeding responds to all of those equally.