If there's nothing actively trying to throw something one way or another, if you were to leave it to nature to let it do its thing and try and balance itself, the positive things would want to move away from other positive things and move towards negative things. But what we find is that this isn't the we have the same concentration of things inside and outside, but the charge is different inside and outside. And that's because we've got active pumps that throw things against their concentration and charge gradients. The relevance of the charged difference comes into play when you talk about those excitable tissues such as nerve tissue.