There is a ton of work that goes into the successful running of an agility trial. It isn't about making money for the individuals that organize the trial. It is about having a great time with friends enjoying the same thing. That said, clubs or individuals that sponsor trials need your help.
Clubs try to set entry fees low but still cover the cost of a trial. But lots of things have to be paid for out of those entry fees: site, equipment upkeep, trailers, gas, judges, $$ to the sponsoring organization, etc. You will not see clubs getting rich running trials.
There are many different types of jobs, some you can do on your own time, and some need to be done day of.
Some of the jobs get you closer to the action.
If you pole set for a group that jumps a different height than you, you get to see the competition up close and personal! You get to sit for most of the runs, just getting up when a pole comes down. I can't tell you how many times I've pole set where I'm so excited and enjoying the run that I forget what I'm doing. Often at a trial you will hear, "VICI! Get that pole!!!" Ah well.
Another fun job is a runner. Leash or scribe runner. You get to watch some great agility and get to know the dogs and handlers better. This is a good job if you feel like you need to keep moving. Some of us are getting a little stiff as we get older. Sitting for long hours waiting for the run only increases the stiffness. Walking back and forth will keep your joints warmed up and have you ready to run when it is your turn.
...but you might learn more about agility
When I first started I was scared to do jobs like scoring or timing. I was lucky enough to have someone encourage me (ok she basically said go sit in that chair and I'll tell you what to do) to try scribing. What I found was by scribing, I actually learned so much about agility. I learned more in that one afternoon than I had taking multiple classes. I'll never forget and am forever thankful. Scribing and timing can be intense. There is much responsibility as you are the key to keeping the trial moving as well as documenting scores. But these jobs are two of the jobs you will learn the most.
Some jobs you do on your own time
There are jobs at every trial that don't require you to loose focus during the day. These jobs include set-up and tear down of rings, grounds management (garbage), hospitality, workers raffle, ribbons, etc. All can be done when it is convenient for you and perfect for the competitor that needs to focus or has multiple dogs running.
The big jobs
The final group of jobs are the jobs that get the trial up and running. Trial chair or secretary. These are jobs that require work before and after the trial. During the trial there are trouble shooting duties, dealing with complaints or problems, or just making little decisions...but most of the work is done before the trial starts.
None of the jobs I've listed are paid. Clubs differ in compensation, but must clubs will give free entries to individuals who do jobs like trial chair, score table, course builders, etc. The jobs that require you to be available when it might not be convenient for you.
Some clubs (SMART and Bayteam) actually will pay full time workers (if you are not running a dog and work the entire day. Full time working is a great way to pay for future entries.
Whatever you do, sitting in the canopy resting is always a good thing, but helping out when you can will help you feel more part of the trial and who knows, you might make a new friend or two.