If the level of technology is high enough that it is feasible to live on a spaceship for generations at a time, it seems reasonable that there would be some people (hermits, political refugees, space hippies, whatever) who would want to live in space full time. Over the course of time, it seems reasonable that groups of these people would form and decide to go live out in space.
One thing to remember is that space is ENORMOUS beyond your ability to comprehend. In order for space itself to get crowded there would have to be an unbelievably large number of people in the universe. People living in space could easily go millenia without running into any other groups by accident. So rather than needing a reason for these people to live in space, you need a reason for them to want to run into other groups, whether that be other space-faring groups or planetary colonies.
As others have already mentioned, one reason for wanting to encounter others is the need for refueling and other resources. The strength of that need would be determined by the level of technology.
Genetic diversity is another important need, both for the nomads and planetary colonies that they may visit. Unless your nomad groups have thousands of people in them, you're going to run into the problem of having a minimum viable population. Wikipedia gives the number as around 4,000 individuals, but I believe that assumes you are allowing the population to grow larger than that. With too few people, you'll end up having a lot of inbreeding and the genetic problems that follow. As such, they'll need to increase their genetic diversity by introducing new genetic material.
Fortunately for your nomads, fulfilling this need for genetic diversity also helps the planetary colonies they visit. As I said, the colonies will also be worried about having enough genetic diversity, so when a nomad group visits a colony it will be a two way trade. Perhaps a few members of the colony want to leave and go somewhere else? They can join the nomads for a while. Perhaps a few of the nomads want to settle down? They can stay with the colony. Perhaps some families will exchange a child with a family of the other group. There would probably also be an exchange of sperm and possibly eggs, whether by sex or artificial techniques like IVF.
How frequently they need to do this will depend on the size of the nomad group and how long they are able to stay on the planets. If there is a group numbering in the thousands (close to the minimum via population) they could go many generations between visits and not suffer genetically. If a group has fewer than a hundred people, they'd probably want to visit a planet at least once a generation. If the group can stay on a planet for long enough for multiple children to be born/conceived, they can get in their diversity needs all in one go. If they can only stay for a short time (less than a year), they will need to visit more planets. There would not be too much benefit in staying on a planet for longer than a few weeks unless it takes longer for the nomads to get the resources they need or they will be able to stay long enough to have a second round of children.
These nomads would also likely stay just beyond the edges of populated space where colonies are just being established. Once a colony has become large enough that it doesn't need to worry about genetic diversity, the nomads have less to offer the colonists and will therefore have to exchange more in order to get whatever other resources they need. If a colony is very small the colonists will be more interested in genetic diversity, giving the nomads an advantage in trading.
In short, there will be people who want to live in space but visit colonies to maintain genetic diversity.