Why do you need to sign a check on the back when you deposit it to your account?
I actually had to go to the bank today and so I decided to ask.
The answer I was given is that a check is a legal document (a promise to pay). In order to get your money from the bank, you need to sign the check over to them. By endorsing the check you are attesting to the fact that you have transferred said document to them and they can draw on that account.
The best reason for endorsing a check is in case it is lost. If the back is blank, a crooked finder could simply write "pay to the order of " on it and deposit it in his own account.
You do not need a signature for the endorsement. The safest way to endorse a check is to write "FOR DEPOSIT ONLY" followed by an account number, in which case the signature is not needed. most businesses make up rubber stamps with this and stamp it the minute they receive a check. That way it has no value to anyone else.
Depositing checks is increasingly going the way of the dodo. Many businesses today use check truncation - the business scans the check in, sends the digital image to the bank, and stores the check. I was surprised that Chase already has an applet for iPhones that you can use to deposit a check by taking a picture of it!
In my experience, you don't need to endorse a check with a signature to deposit it into your account. You do if you are exchanging the check for cash. Businesses usually have a stamp with their account number on them. Once stamped, those checks are only able to be deposited into that account. Individuals can do the same.
I have had issues depositing insurance and government checks in the past that had both my and my wife's name on them. Both of us had to endorse the check to be able to deposit them. I think this was some kind of fraud prevention scheme, so that later one of us couldn't claim they didn't know anything about the check.
I believe the banks are protecting themselves when they "require" your endorsement. Years ago. they used to ask for your endorsement, and not require it. If you endorse the check, it legally authorizes them to debit your account, if the check is later returned for non-sufficient funds (NSF). It mostly protects the bank, and not the customer.