It sounds obvious, but there are actually several reasons why you need to keep close tabs on your credit cards' billing cycles. The first one that should come to mind is you need to know when your billing cycle ends so you can be prepared to make payment in full to your credit issuer, right? That's a no-brainer.
What many people don't think about is how keeping tabs on your credit card's billing cycle can protect you from identity theft and credit card fraud as well. Identity thieves that are able to gain online access to your accounts have learned that manipulating billing cycles and changing contact information is an effective way to throw cardholders for a loop and buy more time to make unauthorized purchases.
They may change the billing or email address on your credit card account so that you no longer receive bills, and if you're not on top of things it may be some time before you realize there's a problem. Make sure you know exactly when your bill should arrive via mail or email. If it doesn't, call your credit card company immediately to ask why.