The Master of Ceremonies isn't alone in the task of organizing seating plans. Most of us have never been to a Royal dinner, so we can only picture the most formal high society dinner we've been invited to and multiply the level of prestige and formality by a factor of at least ten. Being sat with the wrong people at such an event would be both an insult and a terrible social faux pas. It can't be allowed to happen.
There can be hundreds of guests at such events, and so it wouldn't be reasonable to expect one person to undertake all the relevant research to find out who's coming, who it would be appropriate for them to sit with, and who it definitely wouldn't. The Office of the Marshal of the Court assist the Master of Ceremonies with such matters and are colloquially known as 'mini hosts' for the evening.