This is so incredibly important. As a beginner you can basically hit anything often enough and see progress. Once you hit intermediate and the gains slow down you really do need to walk in with a plan. The plan let's you not think and just work. Just working and not thinking, and basically just being told what to do (even if it is just telling yourself) let's you get through the workout repeatably, and with experiencing less of the grind. It also means you hit the body parts with incremental progressive overload in a planned and repeatable fashion, causing growth. And every 6-10 weeks you get the fun of changing it up.

And then there's the days where stuff doesn't quite work, but you have turned up anyhow. Those are days I chalk up to fun.