What is Recovery Point Objective?
If your system breaks, how much information are you willing to lose when your system is recovered? If you only back up once per day your RPO is essentially eight hours. If at the end of the day you finish work and your system crashes and all data is lost, you have then lost the full eight hours of work. If data is lost half way through the day, your effective Recovery Point at that point is four hours, but don’t confuse that with your objective which is set at eight hours.
RPO can and should be calculated differently for different systems you use. Your Exchange Mail Server and SQL Servers should probably have an RPO of 15 minutes to one hour. DNS servers and a static website can do fine with a 24 hour RPO since they rarely change.