Depending on the particular DAP there may be physical size and storage advantages. Furthermore most ‘premium’ smartphones do not come with a headphone jack meaning that the audio experience using a decent pair of IEMs will be an improvement on listening to music on a smartphone via a pair of wireless headphones/earbuds.

P.S. I’ve yet to come across a pair of wireless earbuds that are as comfortable or sound as good as my favorite pair of IEMs.