Rotes aren't Zeal, they're internal prayer cycles that you push Fervour into and they whirr and produce an effect.
Think of Rotes and Meditations and all that like prayer wheels, while Fervour is the thing that flows through them to make them spin. Capacity is how many directions you can subdivide your attention to maintain these without losing track of what's going on in the outside world.
Psyche's "Specialty", is being able to divide attention, just like Hugr is about thinking really hard and sharpening your senses. Psyche is about parallel processing. Each point of Psyche is effectively a another fully functional instance of yourself, which can be subdivided as any human can task manage--maybe a bit better.
Notably, this is also why you don't want Psyche to outpace your Soma by too much. Do it a little, and you'll be fine, it's not like you're putting your full Psyche to use 100% of the time and you can tolerate high performance when you need it--but it's like overclocking a computer--if you wind it up too far beyond what the case can handle, you risk Bad Things Happening.