The real benefit of 80 MHz, IMO, is range with decent speed.
If you just look at MCS index real quick, you will see that, say, with 0.8us guard interval, you get 117 Mbps at an MCS index of 3 for 80 MHz, but you only get 121.5 Mbps at an MCS index of 6 for 40 MHz. It’s just how radio science works: double the frequency range, double the bandwidth.
This means that, in terms of raw speed of the network, you can maintain a decent speed in a larger radius with 80 MHz than with 40 MHz.
Which it mightn’t be required to do, and you should always consider your actual usecase (won’t do much good for VoIP, which wants very low latency as opposed to very high speed). But if, per chance, you live in Europe, where it is quite common to have 500-year-old buildings with walls as thick as those of some high-security vaults, this might be the thing that takes you from “I can maybe read news articles in my kitchen, if I get lucky” to “hey, I can actually watch YouTube videos here relatively comfortably”.