It's not about clothes, it's about signalling preparation and respect. When Bill Gates and Paul Allen were preparing for their life changing meeting with IBM,

It's not about clothes, it's about signalling preparation and respect. When Bill Gates and Paul Allen were preparing for their life changing meeting with IBM, they knew that IBM has very strict dress code, so they dressed in conservative IBM style suits. At the same time IBM executives wanted to make these young unknown west coast guys comfortable and decided to wear relaxed and informal attire breaking their own dress code. When the two groups met, they busted into laughing. It was funny but also made it clear that both were serious and wanted the deal succeed.