What is the benefit of a good transport?

Jitter is the biggie. In non technical terms, it is 0's and 1's, but if the timing of their arrival to the DAC is off or not even or smooth, etc, then the DAC is going to have to work extra hard to turn those messy numbers into music. And down goes the sound quality.

So if it is about jitter is that something that new technology has improved upon or is an old (say early 90s), high quality transport more or less as good as any newer transport?

It is my understanding that jitter can be dealt with at the transport level, the DAC level or both for that matter. A signal is passed, along with the ones and zeros, to the DAC and if the DAC is designed to reclock the signal, jitter is dealt with.

As a non digital expert (not an engineer), how would a DAC know that the signal it receives needs to be re-clocked? I can imagine a transport doing that, since it is reading the source material.

I am no expert either, so those with more knowledge than I feel free to correct me.

The DAC doesn't "know" the signal needs reclocked, if a DAC is designed to reclock it will do this every time. As noted before, the "clocking" information is sent via whatever connection you use, whether a DAC reclocks or not this is passed along. Also, as I am led to believe, doing it at the DAC level is a very good place to do so, addressing it as late as possible, just before the D/A conversion.