For me, it's been key to my recovery & survival. But I can only speak for me. I absolutely agree that it's so very important to separate religion from spirituality & belief.
In my darkest times, I've always known that I'm being watched over and will be pulled through. My mental illness has never been a cause for me to examine or question my beliefs. Perhaps I've been lucky -- or blessed -- in that way. My personal belief system also holds that there are no coincidences; that this is my personal journey; that there's a purpose for my life, altho' there are times when it's hard to discern. This really helps.