If you are truly considering just throwing on an itunes playlist, spotify premium account, or equivalent - then firstly I would truly hope you don't expect much of a party.
I obviously have never been to such a wedding and wish to pretend they don't exsist. But I suppose in certain cases, when dancing or fun is out of the question a simple playlist could be an option...
So okay! I will humour you. Let's pretend you DID use that ipod.
Here are some things to consider:
Whose ipod will it be?
Who will make the playlist?
Are they taking requests?
Who decides what song is next?
If you have a family anything like mine then you can hear the arguments already: Grandpa wants Frank Sinatra, you want some ACDC, and the teens don't know who either of those people are and just want to hear Despacito over and over.
A DJ can act as your music gatekeeper - pleasing the crowd most importantly, vetting requests, and making sure everyone has a good time.
The average 8 hour event requires about 150 songs for 8 hours of nonstop music, and if just one of those songs doesn't go over well - you'll hear about it.
After years of experience I have the ability to read a crowd, sense what songs are working and make decisions on the fly about what to play next and what songs to leave out.
DJ FUN FACT: Songs are all different volumes.
Most songs have different volume levels, especially if you are jumping across genre's and eras. SOMEONE will end up running back and forth turning it up or down instead of enjoying your event, and that someone will probably be you.
Do you have that song?
Do you have all your friend's favorite songs? The frozen soundtrack for your niece? That R&B track your grandmother can't get enough of?
Why spend time downloading (and paying) for these songs you will only use once when you have an entire event to plan!
I have a gigantic library of music that grows with every new request.
Does someone know how to work the Sound system?
What happens if you can't get this sound system you just borrowed from your friend to work or it dies halfway through the night? Are you prepared to stand around in silence or end the party hours early?