There are actually a few reasons for wanting a spring-loaded tensioner.
But first, I'll mention that I'm pretty sure that I am dealing with a TC Bros weld-on hardtail, and it does have an axle slot so that you can adjust tension there. But, that's not enough. For one thing, as soon as the chain starts stretching you end up with tight and loose spots. The hardtail adds a stretch, so the chain will get floppy when that long chain run is at a loose spot. On this particular project, which I am finishing, but did not start, the original builder mounted a TC Bros electronic box. I am anti-electronics boxes, which is a subject for another thread, but since the box is already there, and is extremely nice, what with a matching professional paint job, I'm using it as a junction box. So, the original builder mounted the box low and in-line with the chain. Which means the chain will contact the electronics box during normal chain movement. It can't be helped, but it can be dealt with. I have already added a Delrin Acetal rub block to the bottom of the electronics box. I think adding a spring-loaded tensioner will further help the situation.
I resurrected a hardtail a few years ago, but it didn't have an electronics box. This is a first for me. I'm betting that chain contact is a pretty common problem on hardtails with the TC Bros electronics box.