The words "for themselves" are a metonym for what they will do with the high places. Alternate translation: "built for their own use" (See: [[rc://en/ta/man/translate/figs-metonymy]] and [[rc://en/ta/man/translate/figs-rpronouns]])
This is likely to be an exaggeration that indicates there were many places like these for false worship all over the country. Alternate translation: "on the high hills and under the green trees" (See: [[rc://en/ta/man/translate/figs-hyperbole]])