Why would the GM do this? If you have a player whose goal is for their PC to become emperor, and then right before it happens, you have some other country take over… what’s the justification?
First off, the "right before it happens" is merely unfortunate timing of a locked-in event vs the player's character goals.
“Because it’s realistic” applies to both the takeover and the takeover being denied, so let’s not discuss that. What else could be the reason?
Well, for one thing I want the setting to change and develop in ways not necessarily connected to the PCs, with the purpose of those changes being to provide some variation in the backdrop the PCs operate against. For example, if the PCs have been happily chugging along in peaceful Catallia and suddenly it ain't so peaceful any more then they have to adjust.
At other times, sometimes the big changes are simply long-lasting consequences of PC actions.
An example of the latter from my current game: at one point the PCs managed to knock off the evil Emperor of a particularly nasty nation; and while I always had this in mind as a possibility I never dreamed they'd pull it off so (relatively) soon into the campaign. Result: the power vacuum almost immediately led to a 5-way civil war*, a civil war whose ebbs and flows (barring further influence from the PCs, of which there has since been little if any other than one PC who joined an army unit which then lost every battle it was in) I then had to storyboard out so that if-when any PCs went back there I'd know what was happening when and where and who currently held what parts of the region.
And so, if I've got in mind that (barring PC interference) Catallia will be invaded in the summer of 1087 then a PC adventuring in the 1080s with a goal of becoming Emperor of Catallia by, say, 1090 is probably going to be out of luck. Obviously this becomes irrelevant if the campaign ends in 1085, and just as obviously becomes of very high interest if she somehow gets the throne in 1086.
But maybe the throne-chasing PC will investigate** possible future threats against Catallia, allowing me to introduce or telegraph the planned invasion and thus giving the PCs a chance to - if they want - try to blunt or prevent it; or giving the throne-chaser a chance to change tack and abandon that goal in favour of another.
* - mostly between the slave lords from the A-series.
** - information is valuable, thus if you don't investigate you're unlikely to learn anything beyond the obvious.
I guess what I'm saying is that when people talk about story
now there always seems to be a strong corollary implication of story
here, where I see both relevance and use in having a backdrop that includes and references story
elsewhere.