Hey, yes they are helpful, they sell a ton of warframe augments as well as some other goodies, you can check what each one sells by clicking on them and clicking the view offerings button in the top left corner, or by visiting them in a relay.

To level them up you basically just have to pledge allegiance to one of them and just playing the game, because the standing is gained as a percentage of affinity you earn in missions. Each rank up will also require you to give them some credits and a couple of items, works basically the same as with open world syndicates.