If checks and balances didn't exist, there would be nothing stopping a group i.e. the president, from making whatever laws they wanted.
In essence, if there are no checks and balances, government is no longer a democracy because nothing prevents the group in power from doing what they want. For example, a president that wasn't checked by the opposition party could pass a law saying that they are legally allowed to be president until they die, and nobody would stop them.
