What was the reason for having circular grates over "normal" grates on a steam locomotive? The best I can figure is that they might have
provided better draft through the fire which could have suited the burning of certain grades of coal better.

I recall an offhand comment from an old engine man that you had to fire circular grates lighter than usual, why I do not know.

Are there any locomotives that operate with circular grates today?

Below is an example of the type of grates I am talking about.

I believe a better term for what you are asking about is 'pinhole grates', and more particularly perhaps 'stationary grates' (as opposed to rocking or dump grates).

If this is indeed what is being discussed: my understanding is that there are many locomotives still operating with them, but they are older prototypes and many of them are in Britain (where it was considered normal practice to send some boor bastid in through the firedoor to muck out the ask on the grate or remove significant clinkering that could not be broken up easily).

Rocking or 'riddling' grates were generally considered a substantial advantage on locomotives. Keeping a high controlled percentage of primary-air access in a rocking-grate arrangement led to cast sectional construction systems like FireBar -- which required much more casting sophistication but probably less labor to erect, maintain, and take down than was involved with putting all those holes in a piece of plate...

These are plate grates. Never heard the term "circular". The Strasburg has them on 31 and 90. They are good for really fine coal. Less of the coal will fall through the grates.

They are harder to "shake". They tend to want to dump, not shake. When I was there 30 years ago, they were worn, and we were told not to move them during the day. The risk was we would not get them back to level, and burn off the edges. As a result, we ended the day with the fire right up to the firehole.

I think the official term is "rosebud" grates. I'm guessing that came from originally burning rosebud coal which I recall being a poor quality form of lignite on the Northern Pacific.

With that in mind, all the narrow gauge power in CO uses rosebud grates as does the Great Western 90 at Strasburg. All the narrow gauge engines have burned good soft coal all their existence. 90 did burn some questionable quality stuff on the GW, but does quite well in soft coal today.

One thing one needs to keep in mind when using Rosebud grates is they must be level when you are done cleaning the fire. If you leave them cocked, the air coming up between them will act as a cutting torch and burn the grates in short order.