There are three reasons why helium becomes a necessary addition to breathing gas as depth increases:
Oxygen is known to cause central nervous system (CNS) toxicity at higher pressures. This toxicity presents as convulsions, which will inevitably drown the diver. The current standard maximum safe oxygen working pressure is 1.4 ata ppO2. This equates to 56.5m/185ft when breathing air (21% O2 / 79% N). Below that depth, the fraction of oxygen in a breathing gas must be reduced, maintaining max 1.4 ppO2, by the addition of helium.
Both nitrogen, and probably oxygen, have strong anaesthetic properties at higher breathing pressures. This is defined by the Meyer-Overton Law of Lipid Solubility. As the pressure of breathing gas increases, the anaesthetic effect becomes stronger – causing a debilitating decline in cognitive ability, and eventually unconsciousness. Helium has a low lipid solubility and, thus, a low anaesthetic effect. For that reason, helium is added to breathing gases to reduce the fraction of nitrogen.
As the diver descends, their breathing gas becomes denser. Studies have shown that increasing gas density lowers respiratory performance and reduces capacity to expel metabolised CO2. Higher breathing gas density significantly increases the risk of hypercapnia (CO2 poisoning). The suggested limit, as shown in studies by a huge spike in CO2 retention, is a maximum breathing gas density of 6g per litre. On air, this equates to 36m/185ft. Helium has a very low molecular weight, so its addition to a breathing gas will lower its density. Much of the dive industry ignores this fact, as it stipulating that divers use expensive helium for dives below 36m/185ft would undoubtedly reduce business revenues.