The MG3 is the standard anti-air MG fit on Leopard 2s used by Germany (e.g. these) and some other countries. Some other militaries (e.g. Switzerland, Singapore) use different MGs, though all that I know of use ~7.62mm ones roughly comparable to the MG3, on similar unprotected pintle mounts. The ones in the picture are mounted on the loader's hatch, per standard practice, although they can also be mounted on the commander's.
(The most recent Leopard 2A7 mark replaces the skate mounted MG with an M2HB in a Kraus-Maffei Wegmann FLW 200 remote weapons station.)
Aesthetics aside (I do like the original slab-sided look), the cheek armor fitted on the 2A5 and later also addresses a vulnerability in the protection scheme of previous variants, which is the placement of the EMES 15 gunner's sight in the right side turret frontal armor. The 2A5/6 relocate it to the turret roof, unlike other armor packages like Leopard 2A4M CAN or Leopard 2 Revolution (pictured is a Singaporean Leo 2SG).
