The Romans never started an enterprise without first asking for divine support. They interpreted certain signs to determine the mood of the heavenly powers, and tried to conciliate them by promises and the offering of sacrifices (vota).
Before he ascended to the throne, every Roman emperor took vows that he would offer sacrifices after a certain period of time. In return, the gods granted him a successful reign. This follis from Emperor Licinius (308-324) documents such a vota for a tenure of 20 years (XX).
Incidentally, the Christian emperors made votive offerings, too. However, the vows were not made to Jupiter any longer but to the Christian god.
Not just emperors but also common people vowed to offer sacrifices for divine favors or for godly help. If the request was answered, votive offerings were made. This custom was kept in Christian times.