Lego’s creator decided that every set would be composed of the same set of bricks, allowing them to be combined to produce something even grander.
The financial genius of this idea was simple: now not only would Lego have a brand, like all companies, that consumers would grow to love, but they’d also invest in the ecosystem.
Interoperability meant once child bought first lego set, now they lived in a Lego household. There was more value to be had by buying more and more sets.
See: Autocatalytic processes, network effects
source: Polymatter