Himmler's SS almost became a "state within a state" - competing for resources and having its own military units (some of which fought well). The Holocaust from an organizational perspective, became a way to justify additional resources and control. The process lended itself to delivering measurable, tangible "results".

While there was obvious ideological reasons, the fact that the SS could deliver "results" that Hitler wanted helped.

Germany's war effort was fragmented and, at times, motivated by the leaders competing desire for power and resources. For example, Goering created "Luftwaffe Field Divisions" composed of Luftwaffe personal who fought on the ground and got butchered. Therefore, rather than integrating these personal with the Werchmact where they may better served Germany's war effort, Goering didn't want to lose the numbers for his organization.

The ideological motivations aside, then, the Holocaust while irrational from the perspective of Germany's objectives, was rational for Himmler who was jockeying for influence and resources with the other folks directly under Hitler (Goebbels, Goering)