POI’s, or Points of Interests, are scattered throughout the Starfield universe. Some planets contain more of these than others, some are more valuable and interesting than others. The POI section in the Explorer’s Log is intended to help you save these for future reference.
However, because anything can be a point of interest, the POI section is not intended to help you save just the official game POI’s that you encounter. The intent of this section is to save the locations you find that are interesting to you.
If you’re a virtual photographer, it may be a particular spot on a particular planet that you’d like to return to at some point to capture the rising sun, for example. If you’re looking for the perfect place to setup a home on, POI’s can be used to save your scouting as you travel.
POI’s can, in other words, be used for anything, not just the official game POI’s.