The somatic component was a basic part of spellcasting.
Many spells required the caster to make a motion to cast the spell. If the caster was unable to make the correct motion, the spell could not be cast. Wearing armor or using a shield interfered with the somatic components of arcane spells, creating a risk of spell failure. Bards and some other arcane classes could cast spells in light armor without this risk, because their somatic components were less elaborate.