Why is peer pressure influential in decision making?

some people may change their mind or fashion their opinion around another person's because of peer pressure, like I remember when everyone brought pokemon cards to school, I thought it was stupid! But then when all my mates had some and expected me to have some, I went and bought some with all the pocket money I had which I never would have otherwise done!

...those were the days!

Basically a lot of people don't like being made to feel left out or different, so they will conform or change their opinions, making decisions they wouldn't otherwise make because they feel pressured to.