The purpose of the pyramid is the same as any pyramid scheme.  It's touted as an "investment" but in reality it is gambling.  You are betting that you are not the last person to invest in the pyramid.  If you deposit money to your deposit address, that money gets passed up the pyramid to others that have also deposited.  The more people below you in the pyramid and the more you deposit, the more your possible return.  No deposit, no return!  

So if you just join and think that's all that you have to do to get something then you're not understanding the rules.  Learn to work your referral link so that you actually have a pyramid below you and deposit some btc.  I have 423 people that have joined below me, and more joining below them.  You have to "work" the pyramid.  It's not a free ride.

http://bitcoinpyramid.com/r/2463