routing is one reason, the routing of outboard effects, keyboards, samplers, pre's, etc and not being dependent on a soundcard/interface, many of which really can't past the muster.
another is sound, provided we're talking a mixing desk or console and not just a control surface with faders as that's still mixing with the itb virtual mixer. one of the reasons for the rise of summing boxes (and debates) is that full itb mixes sometimes lack the 3-d sound provided by the real world counterparts. not to mention the over used term, warmth.
unless a daw has been coded wrong, they all sound the same, it's 0's and 1's, same with computers, sound and character are exclusively in the hands of the plugs (after the initial recording of course) whereas real world mixers, maybe even digital ones (not really familiar with them) will provide a 'sound' and it will even vary among it's own series. those neves, ssls, tridents, api's, etc are a huge part of the sound you've been hearing since you first was exposed to music.
i can also tell you, that working on an ssl console actually helped me overstand the plug in versions more because i finally was able to picture it properly. that's just me but thought i'd mention it
my experience is that a hybrid is the best solution. also, why would you see a mixer as something old?