they are stored execution plans, you save the plan in outln user schema's tables (like repository) then you can use those plans for the queries without modifying the sql code, it's a pain in the $%$ when 3rd party software sql is bad and you cannot touch/modify them with stored outline you solve that problem

Actually I think outlines are much better than hints because over the time hints may stop working properly because of the amount of the data and if you want to modify the SQL again you have to talk to developers blah blah etc With outlines the DBA has the control over SQL stmts because the outlines is stored in the database and not embedded in application