A company cannot grow without specialzation. There are a certain number of logical roles that must be filled: design, cleaning bathrooms, testing, sales, accounts payable, shipping, bills, hiring, administration, development, etc. In a brand-new company, one physical person may fill a large number of roles (in the simplest case, the sole founder fills all the roles).
A good analogy is a building made of blocks. Each space on the ground is a role that the company needs to fulfill. The area of this footprint is limited on all sides, so the only way to build is up. The company hires new people to take over some of those responsibilities so the founders can focus on the things that provide the most value.
Each employee provides a certain number of blocks to the company's volume. They can spread their blocks around multiple roles, with limited height, or spend them all in a single column by dedicating their efforts to a single role.
A new employee takes over one or more roles that a previous employee already filled, giving the previous employee their blocks back to reallocate. The only realistic way to go is up, by specializing.
One implication is that you can't create new roles for people. All you can do is recognize where people are overstretched or poor fits for a specific role, and fill in the gaps. Another is that uneven specialization is risky, as it creates spiky towers that are unsupported by the others around them. But uneven specialization allows the highest point on the tower to rise higher than otherwise possible with a given volume.