In the case of my kids a broader musical base on which to base performance. Reading and understanding the accompaniment for instance was already something they were starting to do at 12-13yrs but GCSE has moved this on a bit (and they moved it on further again). You can't really perform properly without a grasp of the harmony to base ornamentation on for instance in baroque music. Ditto sound musical theory is vital.
Kids like mine who perform a lot are slightly perturbed by composition - they'd rather have "the dots" to go on, but the structure and degree of freedom in composition is good too.
Oh and the "delights" of working as a group- they mostly hate this tbh as the standards and intstruments vary widely - French horn cello and oboe anyone? Remember though for GCSE/A level group performance check the curriculum - you may be able to bring in a friend or teacher to perform with.
For a kid who does a lot of extra curricular music and has done some exams actually it's a fairly relaxing choice. And you get marks for recycling repertoire as you don't have to play very hard pieces etc
:)