Your name might seem like a personal issue, and you should be able to call yourself whatever you like — in theory. It doesn’t always work that way, though. You might have a nickname, but when it comes to the legal side of things, only your legal name will work.
Your name is also used for legal and financial purposes, and, therefore, it has some official standing. You may be referred to by a nickname in conversation, but when it comes to official documents, like bank checks, you must use your legal name. When signing a contract, like mortgage documents, using a nickname is inappropriate and makes the contract not legally binding. For legal purposes, your identity is attached to a specific name.