If you lay things out well that it has a much better chance of succeeding than if you lay things out poorly. And knowledge helps with that.

If you optimize more than you need to then that is premature optimization. But if it takes 5 seconds to load a page (or more) then any level of success will kill you and that's bad for business. You need to scale just ahead of the needs of your users. Too much and you're going down the premature optimization rabbit hole, too little and you'll end up with angry users and competitors will likely eat you.