It's a cognitive bias that causes people to think or act in a certain manner because they believe that other people are doing the same.

For example, you could choose to openly display social proof or bandwagon cues, in order to signal to other people that there is support for whatever it is you are promoting. 

• Slow down your reasoning process. Let some time pass between the point at which you encounter social cues and the point at which you make a final decision on something.
• Set up optimal conditions for decision making. Choose to only make a certain decision when you’re by ...

It serves as a mental shortcut that people instinctively use in order to make a decision quickly.