The switch has a lot of similarities with phone hardware, but in a different form factor.
Almost all phones work like this, in that they are mobile-first devices which are designed to depend on the battery.
A major reason for this design choice is power stability.
The switch (just like a phone) can charge off any USB power supply, even really low power ones. The power coming in might be enough to slowly charge, but not enough to keep up when you do the most demanding tasks, like playing a graphically intensive game.
For that reason, the switch requires some charge in the battery, so that if the power draw spikes too much for the charger then the battery takes up the slack and things keep working nicely, rather than unexpected crashing or other oddities.
In the end, demanding the battery has at least a little charge to run is basically a safety feature to ensure that you have a good experience, and the switch does not die in unexpected ways.