Why do blight encounters in Ritual league? or Alva for that matter. Or Bestiary.

Cus I like loot.

The answer is "no, there is no reason to run those if your goal is 36/40 challenges".

they're in there for variety, and to support crafting/SSF when the league mechanic is sparse on deterministic rewards, like this one is.

Sure you can play ssf or just ignore all other content except ritual, but life is better and more interesting if you have access to oils, vorici and hillock benches, corruption altars, and cluster jewels.