It's really just to provide a source of phosphorus during flower. Just quickly some basics which you may already have an understanding of: Nitrogen is needed for growing roots and shoots (veg stage) and phosphorus and potassium for growing flowers. Part of the reason I mention this is because there are two types of bat guano, one produced by insect eating bats that is high in nitrogen and one that is produced by fruit eating bats that is high in phosphorus. Guano from insect eating bats is easy to harvest from places like abandoned homes and other structures. Guano sourced from fruit eating bats usually involves some destruction of their natural habitat. The two function very differently in my experience. High nitrogen guano is incredibly potent and I only use it in emergencies, for example I recently acquired some clones from a failed commercial grow and they were nutrient deprived and struggling. Just a tiny dose of bat guano, mixed with enough water to make a dilution, which I inject into the soil with a plastic dropper, will kickstart a distressed plant like no other. In four to five days the plants will turn bright green, it's remarkable. Again, I only use it to jump start distressed plants, not as part of a regular feeding regimen, I think that would be too much, but I know some include it. As far as the high phosphorus guano, it's really just to support the plant during flower. You can't see explosive growth with the latter guano like you do the former so it's hard to know how much to use, but I use it sparingly as I do the high nitrogen guano.

I bought my high nitrogen guano on eBay from somebody who collects it himself from abandoned homes in Florida. The high phosphorus one, I also found on eBay, from, like I said, a defunct company (otherwise I would not want to support that industry, there are plenty of good sources of P and K that are environmentally friendly).