Simple: DJs are alive and follow the energy and mood of the crowd- playlists do not know what is happening and cannot adapt. If you want your event to have life and excitement; hire a DJ. If you are just looking for background/ambient music, maybe a playlist is fine.
Determine what you want your event to be like, of course we have no control of circumstances like rain at an outdoor event or airport closings for guests flying in, but we can still know what we would like to happen at our event. If you want people to be dancing and moving around, a DJ may be the way to go. This is especially true if the guest list has diversity of age, background, geography, gender, education, economy or culture. Diversity makes predictability of music more challenging since the playlist may not reflect what all the people want to hear.
Example: A birthday party for a six-year-old girl with only her friends from the neighborhood as guests probably will have a tighter playlist than a wedding with guests from Korea, Ireland, Spain and Germany. The latter I did a few months ago and was amazed at the diversity of the music requested to meet the needs of this culturally diverse group of people celebrating a wedding. Truthfully, not only would a playlist on an iPod have been a failure, even carrying six racks of records back in the days when I still played vinyl would not have prepared me enough to make everybody satisfied the way they were. I needed to have the whole 100GB library to be ready for the different styles and periods of music the crowd requested.
An iPod is perfect for a family dinner where everyone will sit at the table afterwards talking and drinking coffee or maybe a pool party with just a select group of old friends as background music for catching up with each other. If you want people to be actively participating in the event; hire a good DJ and make it an event to remember and worth the effort and expense that went into it!
