There are lots of reasons to make positive karma deposits. I wonder if one of them isn't simply the practical benefit of what it does for your imagination.
To make an emotional deposit for someone suggests that you know what would be a positive for them and you can take that (sometimes) vague notion and turn it into a concrete deed or gift. The simple, practical benefit of that? It exercises your mind to think of others. If you live in a market economy, that means you're staying fit in the domain of creating value; whether you're an employee or entrepreneur, that sort of fitness makes you more valuable.
But you didn't really need that practical inducement, did you?