Why do venture capitalist provide finance in stages?

It depends on what stage the business is in and if it is very likely the company will make lots of money. Risk/reward according to the persons knowledge on what the company is doing. Normally there are four stages Start up (from zero), Early start up (a short time trading) , expansion (money needed to expand) and mature (company is trading to near completion and is ready to be sold or trade/expand on its own without any help).