It needs to be an ongoing thing, you have a spine your whole life! The spinal column houses your nervous system, which controls every bodily function. If a vertebrae is out of alignment, it can cause the brain's signals to not reach other parts of the body. Pain is the last sign that something is out of place.
It takes a while to retrain the spine (thus the reason it can hurt worse befote it gets better) and surrounding musculature, then maintenance to keep it in place. It's like Only getting your teeth cleaned when you feel like it but expecting them to stay nice, or getting oil changed when it runs out, or working out once a year and expecting to stay in shape!
A good chiropractor won't pressure you however, but tell you what will work and let you decide what's best for you. Insurance covers most of it these days, and the office I work in at least never charges people too much (too little in fact I think)!
Along the lines of the nervous system, it also helps the immune system stay intact, which is why people who go regularly get sick less often and less severely as well. Plus it's the only thing that works for my headaches and lower back disc problems.