What is the real reason people join political parties?

They're brainwashed into thinking that the parties are different. Sure, each party claims to have all these different views when it comes to the societal issues that they really don't care about (religion, social justice, abortion, gun control).  It's almost as if they divided up the issues to make each party seem really different so the American public could have the illusion of choice.  But when it comes down to it, both parties represent corporate interests and the mega-wealthy and they aren't any different at all.

To support and/or influence its policies. To meet like-minded people.

Of course, where I live there are about 30 national political parties, and half of them are represented in parlement

most of the time it would be because they believe in what that political party stands for and their policies ext. I would say finding a party that you agree with most. doing your research about their past and present would also be a great way to figuring out the right party for your beliefs.

Well by your own addmittance in this question  almost all civillians don't join a political party  they instead just give their alligiance to it in essence by voting every four years to prove their loyalty when it counts

I became a republican along with my friends and family because Democrats said they didn’t want to represent me or my demographic. 

Partying on someone else's money