If there is no mining, how can the node benifit from runing a pow node?
Benefit comes from ability to send transactions.
Basiclly, can you tell how many roles there and how they act and why they join.
One role. Cooperation with peers gives benefits to every participant, this is why they will join.
the motivation for parties to run the nodes is to be able to enjoy a free and efficient marketplace. The parties either have lots of data available to sell or want to buy lots of data or both. But they can't trade their data without Iota because it is either too expensive to trade these kind data in a regular market or there is no place to trade at all. Therefore, the parties will waste their data (income resources) if they don't participate (running nodes because they need to stay online all time to look for trading partners) in a marketplace enabled by Iota. That is the motivation, namely the opportunity to make profit and money. It is the ultimate motivation for business activities.
A not very good analogy is uber and airbab markets. Before uber and airbab, those individuals who participate in uber and airbnb could not have the opportunities to offer their services to other people to make money for themselves because there either no such kind market or too expernsive for them to offer their service in the old marketplace. But with the marketplaces enabled by uber and airbnb they can.
PS - this is why I also like Siacoin so much because Sia is building a similar marketplace for the cloud storage industry. Sorry for trying to sell your another coin.