One of the reasons I seldom do tank journals is that my tanks are more process than goal-oriented--I'm forever fiddling and changing things up, experimenting with new ideas, trying out new plants, etc.
Sometimes it's big--like pulling out half the substrate from my main tank in order to put in sandy areas for my corys, sometimes it's as small as flipping the orientation of a piece of driftwood from horizontal to vertical to improve the 'scape as the background plants grow in. Pretty much every major prune reveals opportunities to tweak the layout.
Having a second (or third or fourth) tank can help that process as it gives you somewhere else to hold extra plants or hardscape ready to hand, or to grow out cuttings for later use, even give you the option of switching out fish populations. This summer I decided to make my main tank all silver/black based on the fish--so my backup tank absorbed my school of aging neon tetras as well as being my main endler breeding tank that supplies the males for my nano tanks.