Its a supplement to GPS, not required for basic auto guidance operations though.  It's only required if NTRIP RTK is used.  Likely the machine has either a novatel or trimble gps receiver as built from factory or did at one time anyway.  Digging deeper into the guidance menus on the armrest would tell you whether one of those receivers is still connected and what level accuracy it is unlocked for (submeter, decimeter, or centimeter).

I'm unsure whether that antenna was only equipped on roof if that receiver was RTK unlocked or if it was standard equipment with only a submeter unlock.