Why will two isps at the same level of the hierarchy often peer with each other? how does an ixp earn money?

When two ISPs at the same level of the hierarchy often peer with each other, then any time they send a traffic to each other they have to do it through a provider ISP, to which they they have to pay for carrying the traffic. The two ISPs may reduce their payments to their provider ISPs when they peer to each other directly. 

Internet Exchange points (IXP) is a meeting point where multiple ISPs can connect and peer together with each other. An ISP earns its money by changing each of the ISPs that connect to the Internet exchange points a small fee, which depends on the amount of traffic sent or recieved from the IXP

When ISP needs to connect with each other then it is done through a provider ISP which is the intermediary and hence the fees needs to be paid to intermediary. When both ISP has peering then the costs that are paid to the intermediary will be reduced. IXP is Internet exchange point which provides this peering facility to ISP and charges a small fees as compared to any other intermediary.

Ibelieve it's the left middle finger