Originates from Metcalfe’s law: the value of the network depends on the size of that network.
Every new user that comes in gives an additional value to every old user on that network.
The way you apply it to cryptocurrency – the more the network grows, the more utility is applied to the network.
So more people accepting/holding/spending, the more utility that network has, and the more value is instilled.
This is Metcalfe’s law combined with a way to value crypto.