A DAP will have its benefits including driving power, additional memory add on, balanced connection, etc.
For sound quality, it has subtle improvements but my simple test is just how long I can listen non-stop for the songs I like. iPhone already provides very good sound quality and with a normal headphone or IEM, most users will not be able to detect any difference in a usual situation of listening on a train or bus. Only when listening to very quiet moments and with high end headphones or IEM, the difference becomes more obvious, though it is still very subtle.
So with the many choices of DAP, iphone, android phones, etc, they are all pretty good in sound quality, and which is the best will depend on what is your preference of sound.
My own test of my "best" DAP is how long I can listen to a DAP without fatigue. X7ii seems to fit the bill, just like my old Calyx M, which is still the gold standard to me in terms of musically and non-fatigue listening for hours after hours.