DACs have a section where they convert digital to analog. That gets fed into another section that boosts that signal to line level, so it can be fed into a preamp or headphone amp.
The amount of power from that output section varies between DACs. People are good at hearing differences in volume and it doesn't surprise me that some pick up on it. As Bigshot said, louder equals better to most.
I think people interpret this difference in output power as a difference in DAC quality. But that's like slightly turning up your amp and saying that it is a different amp.
Level match (with a meter) digital devices and they all sound frighteningly similar.
As for not running everything off the computer, there are practical considerations. I run a traditional setup, with a preamp, separate components, speakers, turntable, SACD player, radio tuner, etc. I don't want to have a computer in the mix. Maybe I'll add a music server for Red Book one of these days, but not today. Second, computers are noisy inside. I don't want my digital conversion picking up RFI from a switching power supply, spinning fan, etc. So I'll use an external DAC, which will fit with my other components.
Amplification is another matter. I like my tubes and, yes, there are actual differences between tube amps. You'll find some material out there about how all amps sound the same and a contest, too. But read the contest rules. You're not allowed to clip the amps and it also excludes most tube topologies. In other words, all amps sound the same except for the ones that don't. Cute, isn't it? After that point is made, they will shift to the argument that these amps are inaccurate. Yes, in some ways they are, but then all amps are inaccurate. And some inaccurate amps are pleasing to listen to.
Also add in the build quality/reliability factor. Well-built amps with a long lifespan don't come cheap. I'd rather spend money on something I can use for the next 20 years than constantly replacing cheap ones.