Pre WW I most armies had 2-4 Heavy tripod mounted (or equivalent) machine guns per
battalion. There were no mortars or light infantry guns in most armies. If a Battalion was tasked with influencing a battlefield (keeping enemy troops pinned in position or preventing reinforcements/supplies from reaching certain positions it could only be done with long range rifle fire. Volley firing was a common tactic. Entire companies firing single shots on command.
A long ranges one heavy water cooled machine gun was worth 80-100 riflemen so giving a battalion a MG platoon of 4 guns was a considerable increase in firepower for a small increase in man power.
In defense a few well sited machine-guns could stop an entire infantry battalion and cause horrendous casualties in a matter of minutes. The answer was lighter more portable machine guns that could be carried during the attack for immediate fire support. by 1915/16 these guns were showing up but often only on a scale of one gun per platoon.
German version
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Much better than a standard Maxim gun but not what was really wanted.
By 1918 more armies had "portable" machine guns and the scale of issue was improving.
However the importance of individual rifle men was declining and in the 1920s and 30s when most armies adopted 80-82mm mortars as battalion support weapons the rifles lost any real need (or even imagined need) for firing at ranges over 1000yds.
British had collected the Vickers guns and kept them in separate units to be attached to battalions or regiments as needed.
Squads rarely operated alone and comparing squad weapons gives a rather distorted picture of tactical thinking. There is of course basic squad tactics but squads were expected to co-operate and work to an overall plan of a higher unit. A single squad might clear a building or small section of a street but other squad/platoons are clearing other buildings and parallel streets at the same time.
A Platoon commander of a 3 squad platoon could very well use his 3rd platoon as a base of fire (primarily the MG ) to support the advance of his other 2 squads which would further break down in advances of the rifle and mg sections of each squad.
In some armies the Platoon commander might have a small mortar available for further support or smoke throwing/target marking (no radios)
It is this proliferation and escalation of "support" weapons" that helped change the expectations of the rifle. Volley fire at 1200-1800 yds was no longer required and even fire at 600yds or beyond could better be handled by LMGs or MMGs/mortars.
In 2012-16 in is no longer acceptable to call down mortar fire on suspected sniper positions in inhabited areas so marksmanship is making something of a comeback.