There's a few reasons for a good fuel log and regularly swapping tanks. First and foremost is to crosscheck that your usage vs the gauges makes sense. Used 30 minutes in a Pa28, the gauges should be down 12-18 liters (or USG equiv), swap tanks and run the other one and repeat the check, have a log to show you verified each time. Then you have in-flight evidence you are complying with whatever fuel burn you planned on, even the latest gadgets with fuel flow gauges showing used fuel/fuel remaining can be wrong and only show feed into the engine or what the engine has taken in, not accounting for things like leaks and so on. You also have verification your fuel gauges are remotely accurate, and some protection against identifying fuel leaks early on. In aircraft with bladder tanks and inter-tank connections/ baffles etc, it's important to check that what you expect to see on the gauge is verified, sudden or unexpected changes (or no change at all, ie using fuel but the gauge stays still) in indicated fuel load could be indications of blockages and problems inside the tanks, that is your tank is not capable of supplying the fuel it's supposed to have in it, if you are not regularly checking the fuel vs a log you will probably miss these things. Then there's the chance of stuff in the tanks blocking ports/filters/breathers and whatever other holes, regular swapping of tanks will mean you catch this problem with fuel still in the other tank, hopefully, if you ran one dry and then the other had a blockage at half capacity, you now have no fuel and it's paddock time.
Lateral balance goes without saying, if you enjoy driving with the brakes on then fly with an imbalance, obviously you don't mind paying for extra fuel and sector lengths.