Since it is always possible to send Ether to a contract, see Forcibly Sending Ether to a Smart Contract, if a contract assumes a specific balance, it is vulnerable to attack.
Say we have a contract that prevents all functions from executing if there is any Ether stored in the contract. If a malicious user decides to exploit this by forcibly sending Ether, they will cause a DoS, rendering the contract unusable. For this reason, it's important to never use strict equality checks for the balance of Ether in a contract.