In this case it is a new version for Apple's video editing application in which they offer us few but important changes. The first thing is as always to improve the stability, security and compatibility of the application and the second thing that they show in this update is the solution of a problem with iPhone XS Max users.
The novelties of iMovie in this new version 10.1.13 are rather out of necessity and it seems that some reports of users with problems in the application make this version reach everyone. At Apple they are launching a good handful of updates these days, the most popular being the new macos catalina from yesterday.
If we look at the notes of this iMovie update, they clarify that the problem with iPhone XS Max users was related to the display of titles to a video in horizontal format. We have no record of major bugs but it is good that they correct all the problems that come up with these types of updates. We already know that iMovie is an application that is available in all versions of Apple's OS and that it is very easy to use to make our video editing.
In short, a series of small but important changes that are always good to improve the user experience and that this time seems to be the case. If for some reason this update does not appear automatically, you can access it through the menu > App Store or directly entering the Updates tab in the Mac App Store on your Mac.