Mushrooms add to the umami flavour, before salt was easily accessible this was probably your best bet for easy flavour enhansement
You wouldn't choose to do manual typesetting (or copying books by hand!!!) today either versus alternatives.
If you're _just_ looking to add umami flavor to a dish today, you'd be crazy to pick foraging for wild mushrooms over Aji no Moto.
That looks like... an intensely conservative estimate. Deer use salt.