Does it give a fixed amount of prestige/signalling/whatever? Firstly, it only does so if you're telling the world about your giving (which, e.g., at least one of the ancient traditions featuring "tithing" strongly discourages); secondly, whether reasonably or not, I think you get more prestige from giving away 10% of what you have if you are either very poor (so that doing so really hurts) or very rich (so that it's an impressive-sounding sum that seems like it can do a lot of good).

However, the fact that a fixed fraction of income is kinda like a fixed sacrifice of utility does seem like it might be an important piece of the picture. Somehow.