EV investment into HP increases bulk, mostly in tank Pokemon, because it allows them to take both physical and special hits better than they usually would. I'll give an example:
252 Atk Life Orb Weavile Ice Punch vs. 252 HP / 0 Def Swampert: 165-195 (40.8 - 48.2%) -- guaranteed 3HKO
252 Atk Life Orb Weavile Ice Punch vs. 0 HP / 126 Def Swampert: 144-172 (42.2 - 50.4%) -- 1.2% chance to 2HKO
While 252 EVs in defence/ sp. defence is obviously better for taking hits in whatever department, that's only advisable for walls where you have 252 EVs to invest in HP as well. For tanks, 252 will go into HP, so it can take both physical and special hits better, and the other 252 EVs would usually go into attack/ sp. atk in order to deal damage.
As for the 128 (not 126) Def/ Sp. Def spread, it's not really a great idea to have mixed walls, since when you spread the EVs too thinly a Pokemon isn't capable of doing one thing too well. That's why you have its team mates to cover its liabilities.