Companies save cash by paying employees in stock instead of cash (and saving cash is just an indirect way or raising cash, a penny saved is a penny earned etc.) Companies cannot pay employees in stock unless the employees have a way of eventually selling that stock. Employees can only sell stock because a robust secondary market exists for buying and selling stock. Therefore, the continuous trading of stock helps companies raise cash and “create value” even after the IPO.

> In fact outside of an IPO you are not investing into a company, but just trading a piece of a pie that already existed.

This assertion is false. The purchase of shares creates upwards pressure on the stock price, which reduces cost of capital.

When a start-up is privately held, they might raise a round of funding to grow their team. Investors get a piece of the company, the employees get paid, and the company gains an employee at the cost of diluting their equity.

When a publicly traded company wants to grow, they can give the employee stock. The employee can turn around and sell the stocks on the market. As above, investors get a piece of the company, the employee gets paid, and there are more outstanding shares.

I'm not OP but I think their point was that the two cases are not so different. The line between what is "trading" and what is "investing" is not so easy to draw.