In the US IPOs are the preferred way for Venture Capital firms to unload their 'stake' and take a profit. It is also a way for the principals of the company to get a 'pay-back' for the work they've done.  Third, it is almost the only way, in the US, employees of the company actually get some economic benefit of the work they've done in relation to the "wealth" or "value" they've produced.  Somewhere down in the basement is getting money for the company to invest in current and future operation(s).  

Have to be a damn fool to buy into an IPO but there seems to be plenty of damn fools running around ... and we know what happens to their money!