From my understanding these substrates have high CEC (cation exchange capacity), i.e. the substrate is able to hold onto ferts that you put into the water column and since the substrate is holding onto these ferts it makes your substrate more fertile. I don't think it's indefinite, but I think that it kind of acts as a buffer so if you were to not fertilize for a few days your plants wouldn't get hungry. At least that's how I understand it.
Plus I think flourite looks nice for whatever that's worth.