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A spaceport is a way to increase the volume of interplanetary or interstellar transport.They can be locate inside space stations but the focus here is on planetary ground stations.Note that if the booster vehicles are pathetic and weak (like current NASA chemical rockets), the spaceports will have to be located at specific lattitudes.
Their primary function is to do whatever is necessary to make it easier for spacecraft to bring cargo and/or passengers.
They may be located at an economically worthless site which happens to be at a strategic location (cross-roads or junction). Please note that increasing the volume of transport does not necessarily mean transport.
A spaceport created by a large corporation might facilitate transport spacecraft belonging to that corporation, but charge large fees and otherwise put roadblocks in the way of ships belonging to rival corporations or to independent ships.
Note that the concept of a "junction" really only works with some kind of handwaving faster-than-light starship.
Junctions make no sense in a star system where the various planets orbit at different rates. As with spacecraft and space stations, in a science fiction story a spaceport can become a character all by themselves.