Being a top witness (in the top 20) means two things: greater pay of steem for being a witness, and more importantly, the responsibility of being part of the "consensus set" of the Steem blockchain. What this means is that you are responsible for ensuring validity and correctness of the blockchain.
While the responsibility is mostly automated, its important to not let these positions become corrupted or vulnerable, as they are like a form of governance.

Positions underneath don't have the same responsibility, but might become elected by getting voted in, so they should prepare for it. Positions under 20 are paid far less steem.