So that when a teenager types "fortnite" in on their phone they can install it. That's the only reason. It's a little different from organic "discovery", like search and recommendations.

FWIW, Google (illegally? according to Epic, anyway) blocked Epic's attempts to address this through partner deals with phone manufacturers, so their hand was relatively forced.

If Google and Apple didn't require all installs to go through a blessed path, people would just type "fortnite" into their search engine of choice and install that way instead. Of course, Google & Apple have good reason for not allowing that, but in the case of Fortnite they really are not delivering much for the 30%.

Epic still allows Fortnite to be sideloaded.

More importantly, Google does not prohibit sideloading. It simply does not make it convenient to do it.

I suspect the OP is referring to the fact that Epic is also suing Google, because they consider the "inconveniences" that Google places on sideloading to be a form of unfair, illegal restriction. This strongly suggests that if Apple offered sideloading but still had the same fees and restrictions in the official App Store, Epic would still be suing them.

Stores do hold a value. A great one. BUT there has to be a few tiers between we are really losing money by hosting this to we are raking in hundreds of millions. If should cover cost, it should cover opportunity, it should have a margin.

In essence Apple is running an Apple App incubator :) Getting 30% of anything, anybody ever makes on it, with basically no risk on it since the actual cost of reviewing the apps and hosting them is pretty fixed.