We only use that to allow changing the layer tag from CSS rules so far.

This allows fixing buildings placed on two high layers above their
interior for a number of major railway stations for example. This is
caused by the layer tag being (ab)used as a z-inded for the outdoor
renderer there, but not being consistently applied to all elements.

In those cases we cap the layer to 0 now. More such rules are likely
needed though, there are more scenarios where layer tags interfere.
But that's much easier to extend now that this can be specified as a
simple CSS rule.