>>214219482
They keep upstart politicians in their place by monopolizing being snobs and aristocrats WITHOUT having any actual hard powers*. The last point is important. When commoner politicians rise in power in Republics, they tend to establish dynasties that over time either turn to dictatorships or ineffectual bureaucracies. Having a constitutional monarchy reminds the politicians that they are replaceable and serving the country (instead of ruling it). Tldr: Royalty get to be the posh buffoons so that you don't get a Trump.
*Technically in the Westminster system, the King can appoint and fire a PM and dissolve the Parliament but tradition and self preservation dictates he only does this after a General Elections or at the advice of the PM.