A watch's primary function is to tell us what time it is. This is why telling the time is not considered a "complication".

But anything beyond that is.

And some complications are much more difficult to make than others. And combining more than one of these complications into one watch is absolutely spectacular, hence, it's own category: Grand Complications (minimum 3 complications to qualify).

But even doing one of these things well is a challenge, and making them all, even in individual watches make for quite the repertoire.