Fly ash is - generally speaking - an exceptionally good additive, its use is essentially due to the size of its particles, you can consider it as a very good filler with some added value (and it has some very useful side-effects, namely it makes concrete much easier to pump).
Besides the raw resistance, you have to imagine concrete as being a sort of artificial stone, the idea is to fit into a given volume as much material as you can, and you obtain this by mixing together gravel (usually one, two or even sometimes three sizes), crushed sand (rather big in size) and (where available) natural sand (or more finely crushed sand).
Then you add the cement, which is the finer "powder like" material, and water, BUT in many cases the "granulometric curve" remains "empty" in the lower part, and there are practical limits in the amount of cement you can put in the mix, so you need to add something (a filler) that is fine as or finer than cement, and this is often fly ash, which while not being as powerful as cement as a binder has anyway a pozzolanic effect, that helps in reducing the permeability of the set concrete, the matter is briefly explained in the second part of this:
Besides the raw resistance, you have to imagine concrete as being a sort of artificial stone, the idea is to fit into a given volume as much material as you can, and you obtain this by mixing together gravel (usually one, two or even sometimes three sizes), crushed sand (rather big in size) and (where available) natural sand (or more finely crushed sand).
Then you add the cement, which is the finer "powder like" material, and water, BUT in many cases the "granulometric curve" remains "empty" in the lower part, and there are practical limits in the amount of cement you can put in the mix, so you need to add something (a filler) that is fine as or finer than cement, and this is often fly ash, which while not being as powerful as cement as a binder has anyway a pozzolanic effect, that helps in reducing the permeability of the set concrete, the matter is briefly explained in the second part of this:
https://en.wikipedia.org/wiki/Pozzolan