Just one of the many decisions the designer makes.
Not really relevent to 'trainer aircraft' per se.
On/Off: simple but controlling a fuel imbalance is awkward.
Tank selector: Have to change tanks but then a fuel leak in one is isolated from the other.
Also the general system or some parts might be used across, or have evolved from, a variety of models, with various combinations of tanks. Easier to adapt if some semblance of multiple selection was already built in, or not worth the bother if it was there already.