Why are business partners, although it is possible to put money in banks ?

business partners are investors in company's business who expect a certain rate of return on their investment over the life of the company while meeting other objectives like generating employement, building reputation, creating base for business diverfication, building brand awareness, image for their product and service

Your question isn't clear enough but none the less i will try contributing something.

One may prefer investing their money in partnerships because they are interested in "more business" & "more profits". Partnerships will surely guarantee you more & "bigger" profits compared to the interest you get when you bank the money in banks. Banks provide interests on money banked with them but this money or interest is less compared to the money or profits one would get if they invested their money in partnerships with other business partners.

Well, We are human....so..the bank means more trust, from both sides to eachother..and more control.