It's somewhat of a BS faux pas/custom that I personally think is complete crap. The general argument is that if you're going somewhere that requires you to wear a tie, it's probably upscale enough to require a suit or jacket as well. So if you're going to some sort of event that requires a tie like presenting at a medical conference, then yes, you should generally be wearing some form of jacket or coat with it to have a finished look. If you're required to wear a tie to work or school as a mandatory thing, then I think the whole "you should be wearing a jacket" argument falls apart. I never saw the point in wearing a jacket when I plan on literally taking it off for the entire day and not wearing it unless it's cold enough outside to warrant wearing one. Even then I usually pick something warmer over a sport coat.
I'll also add that generally speaking when I have to wear a tie but don't wear a coat, I roll my sleeves to give a more casual look since laid back is more my style anyway. If an event is formal enough that something like that wouldn't fly, then it's likely formal enough that a coat is required anyway. When I say rolled sleeves, I'm talking just at the forearm like the following, not all the way up: