Founders take common shares when they create a company, and in the "big win" scenario, this is what powers wealth creation for founders.  However, there are many "in between" wins, which are not huge, but do result in nice returns, especially for investors.  However, in some cases (particularly in down markets, something few people seem to remember right now), only those who own preferred shares get the return, due to preferences and other terms of the investment round. 

A smart entrepreneur will find every opportunity to exchange cash for preferred shares - not common shares.  If you are going to put significant money is (say 10's or 100's of thousands of dollars) - then put it in "alongside" investors.  One way to do this in the early days is to put in a convertible note - a pretty common structure that converts alongside the first investors at the sames terms that they get.  This prevents any "premature valuation" of the company, and is viewed as fair by investors (the founder put in CASH just like we are), as well as other founders.  Some founders will say that they get far fewer preferred shares than they do common shares (common is usually 1/10 the price of preferred shares), and this is true.  But the preferred shares are those that WILL ALWAYS get compensation before the common shares. 

Hence, Preferred Shares are the cake, and common shares are the icing.