Now why would an army need mountain guns? Let me think? To use in the mountains? Imagine trying to set out a line of infantry in the mountains. - once you have got the regiment up there. These small guns were VERY useful in country that did not suit soldiers, never nmind vehicular traffic. They were portable, often made to be broken into easily managable parts for the crew and the ammunition was easily carried too.They could shoot ACROSS valleys without too much difficulty AND cause damage too. The shot could be anything - ball, shell, canister and they would also hit the rocky bits and send shards and splinters everywhere. They may not have been spectacular, but would certainly put the enemy off more than a bit. They were also used in jungle terrain too, for the same reason.
BTW - the old 2-pdr illustrated was a very successful anti-tank gun and did quite well until the Nasties up-armored their tanks after 1940. It was effective out to 1,000 yards. It was very successful against fortifications and Italian and Japanese tanks too. The carriage allowed 360 degree traverse and , even though hand-loaded, could fire 20 rounds /minute. It was also adapted to be main turret armament for most early war tanks and armored cars. Its main disadvantage? AP rounds mainly, no HE rounds.
Bringing the