Elementals were the first power armor units that were seen in battlefields. They were a Clan invention (Clan Wolf, to be precise). The Clans bred, through their artificial breeding program, warriors specifically for power armor warfare.
Give 'em almost impenetratable, self-fixing armor plating, jump jets, a detachable SRM-2 launcher, laser and machine gun, and you have a fearsome opponent that can take out a lot of unarmored soldiers. A point (squad of five) of these armored warriors can also be dangerous against light BattleMechs!
Victor Ian Steiner-Davion thought, when first confronted with these things, that they had been attacked by an alien race. He called them "toads". Of course, later it turned out that they were just humans (with admittedly somewhat alien ways of thinking).
Elementals were, for long time, the only armored fighters on the battlefield. As with OmniMechs, it took some time for Inner Sphere to get on to the same technological level.