Preamp - what's the purpose?

Intentionally dumb question...

I've heard various 5-15W tube amps in my room. EL84, 300B, etc. They all have input stages and the output stage. I send them a line-level signal from a DAC.

Sitting a few meters away from my loudspeakers, the first watt alone gives me roughly 80db of volume. I think these amps are biased to expect the line level signal directly. Why wouldn't the designer do that?

So what's the point of adding a pre-amp? Why do people do it?

thanks in advance