In addition to what others have said: an opportunity to invest. There are plenty of great investment opportunities around, but not all can be had at the click of a button.
It makes it easy for an investor living in France to invest in a US tech company.
1. It offers owners and/or founders of the company liquidity, which is essentially why they started the company in the first place.
Just because you own an amazing company doesn't mean you want to keep your entire net worth in a single stock.
2. I would buy Apple shares if I could sell them at a moments notice. I wouldn't if I couldn't.
3. A liquid investment is generally valued higher than an illiquid one.
4. A higher valuation means the company is able to raise funding from the market at a later stage, by selling additional equity (if required) -- this is relevant for the company.
We wouldn't have to call it an Initial Public Offering if companies never sold shares in subsequent Public Offerings. Most trades don't have the company in question on the selling side, but some do, even after the IPO.
It makes it easy for an investor living in France to invest in a US tech company.