With today's tech advancements, it's quite easy to make playlists on your phone/computer/mobile-device which contain every song you love. So why would you want to pay someone money to do this for your wedding?
A DJ can cater to ALL of your guests
A good DJ can read the crowd and react to the differing moods throughout the night. Sometimes the atmosphere can change quite quickly without any notice and a good DJ can react accordingly. For example, after your first dance, the atmosphere may be quite and your DJ will play some slow waltz love songs to cater to this. This may only last for 2-3 songs or may continue for longer. If you were using an iPod, you could go for too long and will have to change the song halfway through (not mix it out like a DJ) and disrupt the mood.
Chances have it, your music collection won't compete with a DJ's
Majestic Wedding DJ's has over 85,000 songs and counting on their playlist. With a mountain of songs available, you would be certain that 95% of requests throughout the night will be filled. An iPod can only carry a certain amount of songs, and chances have it, you won't want somebody looking through your iPod trying to find that old 90's RnB song from your childhood years.
A DJ can 'drunk friend proof' your music selection
Throughout the night song requests can get more and more ridiculous. Having a DJ ensures that any cheesy or banned songs do not get played. When you allow an iPod to be accessed by all of your guests, you are playing with fire. We all know that one person who decides to listen to the first ten seconds of 30 songs before they find what they are looking for. Trust me, you do not want that person at your wedding.
There's no substitute for quality. Hiring a DJ for your wedding ensures that the music quality (bitrate) is good, the equipment/speakers are suitable for the venue size, and the volumes are mixed correctly. Unless you have a sound technician as a guest, you will find tending to the iPod will become a constant task throughout the wedding.