Bear Cavalry are the main Polar Dwarves melee unit, necessary to stop aggression, and necessary to have as a threat that can come out of your Ice Golems.
Bear Cavalry make it very hard to attack your Frost Mages at melee range for an enemy common. This is because the Frost Mages are usually next to a gate and if the Frost Mage dies, a Bear Cavalry can replace it and be immediately ready to Trample the melee unit that killed the Frost Mage.
They fill a Svara weakness, in that having many gates (especially in the form of Ice Golems) can be nice, but only if you have something threatening to come out of them. The Bear Cavalry fills this role of “nasty threat” that can pop out of your gates. In the proper situations, a Bear Cavalry is the best way to deal with enemy melee commons. Even if they don’t come out Trampling when first summoned, they have enough life that they are likely to survive, and then can Trample whatever common was used to attack them at melee range.
They are also excellent in offensive pushes as they can Trample through blocking enemy commons to unexpectedly arrive next to the opposing summoner.
Bear Cavalry are also the best Target of Frost axes, Bear Cavalry + Frost axe is the main way you have to threat enemy Champions and Summoners.