Right now, I'm listening to iTunes via my new MacBook Pro 2017, with my Sennheiser HD630VBs plugged directly into the headphone jack. There is no background noise or hiss. When I pause playback and turn the volume all the way up, there is zero change in the background.
Ordinarily, my home setup is a Schiit Modi2/Magni2 Uber stack. However, unlike a lot of people, I didn't get it for the "extra oomph" or the "better sounding DAC". Until my two Massdrop Senns arrive in the summer, I only have easy to drive headphones--I can easily deafen myself with my current crop with my iPhone, let alone a dedicated amp. But, I have multiple sources in my home office, a few of which have no direct headphone jack, so the Schiit stack allows me to switch easily among the three sources (a BD player hooked to an external display, a region-free DVD player, also hooked to the display, and my laptop). Much easier to switch sources with the stack.
I expect my new phones will benefit from the amp, at least, given their more demanding load--especially the 6xx. But the DAC, to me, makes no difference in sound quality--either from my old 2009 MBP or my new one. And each of them is dead quiet with my headphones (currently, I have the HD630VB, B&W P5 s1, Grado iGe iem). Perhaps I'm lucky. My wife's laptop is a Lenovo (I think--I don't use it except on very rare occasions) and it is a bit noisy through the headphone jack. If that was my laptop, then I'd have gotten a DAC even without the other reasons noted above, if only to minimize the noise.
Ultimately, I've not been able to discern differences between DACs at level matched comparisons in terms of sound quality. But to deal with noise issues, or for the convenience of multi-source setups, they can be great (mine is perfect for that). I would like one with a digital OUTPUT, though, as I still like to dabble with my now ancient Sony MD deck. Been thinking I'd add a Cambridge Audio DacMagic Plus to the main system--would give me headphone alternatives for late-night listening and has a digital output to accommodate my eccentric audio format. But that's for another shopping spree.