Why does it need to be successful in terms of readership?
Because nobody wants to write something that nobody ever reads. That's kinda like asking, "If you're not going to sell that painting for money, why bother making it look good?" Uh, because I want it to and I want the people who see it to enjoy it.
The painting might be for your mother. If so, you'd make it something she would enjoy, right? That's an example of knowing your target audience, even though you're not making money.
What sort of difference are we talking?
I mean - I talk corporate bollocks because that's what I do for a living but I find it frankly sad that people are limiting their creativity because they are worried that 42 people rather than 48 will read their fan fiction.