On the other hand if you offered to buy them a beer or a coffee it would probably be very motivating. I'm not sure why this is. Maybe cash just feels lazy and impersonal, so the amount being offered has to be big enough to counter that feeling.
That is certainly a plus, but I don't think that's all of it. If I was working on something for a friend, that they had no idea how to help with. Then at some point they dropped off a snack or drink as a thank you, I would be happy about that. If they offered me a $100 bill I would be offended because they're not my boss, and my time is worth more than that.
I want to say that a small token of appreciation feels better than having my value quantified to an insulting amount. However, it may be even more basic than that. Food is a powerful reward, there is a reason it is used to train animals. It also could be that introducing money makes something feel like an obligation.
For anyone who is working, people are "giving" them $5 all the time. But they probably don't have people buying them coffee/beer/lunch all the time.