The original purpose for operating Field Day was to operate under simulated emergency conditions. Many participants use generators, batteries and solar power during the twenty-four hour period. Also temporary antennas must be erected. A secondary purpose for Field Day operations is that of contesting. Most participants try to better their score.
Some groups use only one transmitter while many others use several.
A Field Day operation also gives everyone a chance to try new bands or modes. My first and only satellite contact was during the Field Day event several years ago and I really enjoyed it. I also recall my first ever contact on six meters was during Field Day. For some licensee, Field Day may be their first experience operating the HF bands. Perhaps the best reason for coming to Field Day is fellowship with other club members.