Why we need to use Odd number of nodes in MongoDB ReplicaSet

In MongoDB ReplicaSet, the voting is done by a majority of voting members.

Imagine a Replica Set with three (voting) members. Let's say that Node A is primary, and nodes B+C are secondaries. Node A goes down, so nodes B+C go to election. They still do form a majority (two out of three). The election is first decided by priority. If both Nodes B & C have the same priority, then the one who is most up to date in respect to the failed primary (oplog) wins. Let's say it's Node B.