WALL OF TEXT INCOMING
TL:DR Cos it's better (for me)
I have a Pioneer XDP-100-R DAP (Amazon Link) I purchased recently. Before that I used my phone (Samsung S5) and hooked it up to my work van's stereo via aux cable and it worked fine. I'd probably still be using that setup except that
a) playing music for 6 hours straight drains the battery and I kinda want my phone charged so I can use it
b) playing while charging over long periods can't be good for the battery, it certainly seems like it doesn't hold as much charge and drains quicker but that's likely just anecdotal
c) playing while charging adds audible line interference from the charger, a constant wheeeeeeeeeeeeeee in the background that gets louder the harder the engine's working and changes pitch when the van shifts gears
d) using a ground loop noise reducer thingy is yet something else I'd have to carry around with me and hook up in addition to a charger and aux cable
e) on some old quiet tracks I'd have to REALLY up the volume on the S5, pushing it past the EU safe listening limit, which seemed to really drain the battery like there was no tomorrow so I ended up using a portable amp which is yet another thing to carry around with me and hook up every single shift
f) the places I often drive to don't exactly have good signal for streaming, etc, so I have to use a microSD card (which I'd likely do anyway because why stream and waste data when I have the music I want anyway) but phones only have 1 microSD card slot and I easily filled a 200Gb card - I despise changing the contents of a card/player with a passion bordering on insanity. I want all my music with me everywhere I go so I can listen to whatever I feel like without having to predict my needs or arse about copying and deleting and aaaaarrrggggghhhh just thinking about that pisses me off still.
g) I also don't want to rely on an outside party allowing music to remain accessible and available. If they, say Spotify, decide to remove an artist then I'd be screwed if I wanted to play them. The Pioneer has 2 microSD slots for 432Gb total storage (less what's used by the Android OS and apps) which allows me room for everything in my main library with room to spare, for now
h) I still need to toss around on my phone playing games and whoring reddit on my breaks while listening to music, that's easier with a dedicated DAP
i) I just don't want to use my phone for everything, personal taste-wise I'd rather have a dedicated music player.
I literally spent around £500 that I didn't really have spare on the Pioneer and a couple of sets of decent IEMs solely because I wanted another microSD slot that no phone has (that I'm aware of (that actually exists - I'm looking at you, Saygus V2)). I'd not have spent money on a DAP with low storage capacity no matter how good it was, that would defeat the primary point for me. For me it was either the Pioneer or the iBasso DX80 and I went with the Pioneer because I was already familiar with Poweramp and could keep using that rather than use the iBasso proprietary software. Plus the Pioneer is virtually the same size as the S5 and fits perfectly in the van's built-in dashboard cradle (which is intended as a mount/charger for the company handheld scanner/satnav/phone device but screw that, my DAP's more important and I have my own satnav anyway).
That's not even touching on just how much better music sounds played through the Pioneer rather than the S5. Even through my work van's crappy stock sound system there's a drastic improvement in sound quality using the Pioneer. The Pioneer allows me to hear musical subtleties I didn't even know were there. It's far better than the S5 or my laptop and current headphones in that regard, which is why I'll end up grabbing a DAC/AMP combo and some new headphones when I get chance.
While I'm sure there are some great phones out there when it comes to music playback a phone is a jack-of-all-trades device where as a DAP is, well, dedicated. The entire thing is optimised for one thing, software and hardware, and it performs accordingly. Being an android-based device the Pioneer would still allow a user to use any android app they wanted to, and the output would likely sound better because of the dedicated hardware. I just use Poweramp though. The only thing you can't really do with it is call people. Then again I have a phone for that. :)
Since I'm using the Pioneer at work I don't need wifi active and on 6 out of the 7 vans in the fleet I don't need bluetooth active (one van has a dodgy aux jack so I have to bluetooth it there). With that off I can get 16 to 17 hours or so of continuous playback on one charge, or roughly three shifts. I'm not going to get that on a phone, ever.
In most cases in most circumstances for most people a phone would be easily good enough, I will concede. It was certainly good enough for me until it kinda wasn't. It all depends on personal preference though. If you don't want to carry around an extra thing then that's cool, whatever works.