A reading child can begin to write and type earlier. They can be exposed to complex computer use instead of simplified apps. They can use more advanced materials to learn other skills earlier. They can gain a ton of knowledge from the extra years of reading. And even if they cannot understand much at 4 (which of course depends on the kid), they can at 7 or 8 - being fluent readers by then gives them the advantage of consuming many extra books and richer ones.

There are also psychological components. Until school starts the parents have significant influence over 'cool' skills, important values, 'fun' things to do, etc. Schools rarely breed love of learning and peers often push away from it. Learning at 7 creates risks of it being perceived as less cool or being consumed by other activities and not catching. The time before school is hugely valuable imo. And reading earlier gives the parent another tool to teach by providing books.

It is true that the earlier you start the more time teaching takes. But this time isn't only teaching reading. It is excellent bonding time if you don't turn it into an authoritarian war zone. It teaches focus, overcoming obstacles, etc. On the whole, I think it at least holds its own with other activities parents spend time on with their toddlers. The bulk of the challenge is getting the kid on board. Buy-in and motivation matter far more than any specific method since without them the risk of the effort hurting the relationship or their love of learning is too high.

I used a method of my own making. It probably wasn't the most efficient, but it was always tuned to the kid. My oldest was fully decoding in late twos and reads a ton now in late sevens. He naturally memorized words and we added phonetics later. My youngest is decoding short words with basic sounds in the early threes. He is not a fan of memorizing, but took to phonics naturally.