The space beacon is a rarely used option in AAT. It is used to leave notes in space sectors. This can be used for notating ownership, trade routes or other useful notes.
Don't plan on using these beacons as a permanent message. They can be changed by anyone that clicks on them.
This fact can make them a very interesting billboard of messages back and forth between sector owner and anyone that enters the sector. Warnings and CLEAN trash talk make them fun and at 100 credits they are cheap enough just for entertainment value.
One game Tarnus made a scavenger hunt using them as direction billboards. While this is fun, be sure that it is not leading you into a trap. Try spicing up your game with a few.