Why do BIG companies have a stock market?

ItzUrBoyKeith asked:


This morning started playing a stock market game as practice to get used to the stock market and how it works. However, I don’t get why HUGE multi billion dollar companies like COCA-COLA or MICROSOFT need a stock market. Even GOOGLE, being a search engine holds very high stocks. Why is this necessary?

Thanks in advance. I know it’s a very elemental question but I know nothing about economics.