Portals are wormholes to other planets located closer to the center of the galaxy, they do not work like black holes do, as entering one allows you to travel immediately back through it to the planet from which you came immediately. These are not recommended as a means to travel closer to the center of the galaxy permanently, but instead as a means of financial accumulation. Planets closer to the center of the galaxy are more likely to be rich in materials that are useful to sell. Couple this, with the fact that the Galactic Market in that system will likely be deficient in many of the resources found in that system, you have a recipe for financial gain. Be careful, though, as better resources and closer proximity to the center of the galaxy usually coincides with a more hostile and advanced sentinel force, as well as more dangerous creatures.