John Gill Commentary 2 Chronicles 12:8

"Nevertheless they shall be his servants, that they may know my service, and the service of the kingdoms of the countries." — 2 Chronicles 12:8 (ASV)

Nevertheless, they shall be his servants
tributaries to the king of Egypt:

that they may know my service, and the service of the kingdoms of the
countries ;
the difference between them, how easy the one, which they might perform without taxes and tributes, and how hard and heavy the other, through the exactions and exorbitant demands of those to whom they became subjects.