Many people don't bother checking their bank statements because they trust their bank not to make errors. In my experience this is a bit optimistic. I check my statements and have found errors. In one case many years ago my bank failed to record a deposit. Although it was a fairly significant amount of money (2 weeks pay if I remember correctly) I might not have noticed if I hadn't tried to reconcile my statement.
I recently discovered another error. It is a bit annoying since it is too late to do anything about it and it cost me $100. I have been entering old data into Quicken and found in 2006 I apparently failed to add up several items in a deposit correctly. So the deposit was recorded as $100 less than it actually was and the bank failed to correct the error. Note simply checking that the balance on my statement agreed with my records was not sufficient to detect this error. I wonder whether it was the bank or the teller who received a $100 windfall.
Of course another reason for checking statements is to detect other problems such as erroneous or fraudulent credit or debit card charges or forged checks. I haven't encountered these but they seem to be fairly common.