Production companies are ephemeral and tour multiple locations. Depending on the production they may require different sorts of equipment to accommodate different performance skills. It's totally unfeasible for a venue to stock a wide enough menu to satisfy a variety of productions, and likewise totally unfeasible for a production that shows up and has mere hours before showtime to adapt to someone else's instrument rigs beyond the basic necessities of PA. You can rent but it's often better to buy as cheap as you can to eliminate complexity and risk.