Mahler's Tenor Horns

This grade 11 girl went to the Mahler 7 concert recently, equipped with the score and friends with the score.

Girl [in lesson]: I just don't understand why Mahler uses the tenor horn there.  It makes no sense.  You can't hear it... wait... tenor horn? [she pulls out her phone and checks the score on her phone - yes, really]. Yeah.  Tenor horn.  I mean, really!

At lunch, she barged into my room.

Girl [quite excited]: I had a realisation!  I worked out why Mahler uses the tenor horn.  It's for [pauses slightly] Dramatic Effect.  .... Yeah!