I’ve always heard it was to make conversion to Christianity more palatable, full stop. At least in Western Europe most of the conquering was done before Rome was Christianized, and they never conquered the Germanic peoples who are the pagans who give us most or all of the familiar Christian trappings (Yule logs, Christmas in December, the word “Easter”, Christmas trees, elves, etc).
The pre-Christian Romans made conquering easier by bringing Roman luxuries (baths, food, wine, commerce, citizenship, etc) to conquered peoples and by making examples out of those who resisted, but Christianity was not part of the package.
Not sure what the Eastern Roman empire was doing with respect to conquering and Christianizing, but if they converted anyone, I don’t think those pagans contributed imagery back to Christian holidays as we know them in America.
That said, after the Western Roman empire collapsed there were lots of European powers who used conversion as part of their larger subjugation toolbox.