Rule of Animation Conservation

"Any animated work or animated part of a work is animated for a reason."

The Rule of Animation Conservation (AKA Pencil Mileage) stipulates that, if a work is made through any type of animation, something about the story or the characters has to justify using the medium.

This is because animation is (at least, up until recently) relatively expensive. Sure, it looks pretty, and it's fun, but it isn't practical to blow extra time or money to make something animated when it doesn't have to be.

For example, suppose you make a big budget CGI movie, or even a movie that contains a lot of CGI, that's about the life of a perfectly normal human family in which perfectly normal stuff happens. This film could easily have been made in live action. It would have been cheaper to make it in live-action—and the less expensive the film, the fewer viewers it takes to make it profitable. And using animation for this kind of film increases the risk of Special Effect Failure and decreases the realism; either of these will annoy some viewers... But, if you want to make the movie CGI, you can turn the family into, say, superheroes. It can still be done in live action, albeit with lots of special effects, but now you have justification to make it all CGI. Just remember the superheroics.

It is easier to justify an all-2D animated film than a film that's all CGI animation, especially if the film has a human cast. That is because using CGI to depict humans realistically will trigger the Uncanny Valley. Hand-drawn humans, or even 2D machine-drawn humans, don't have this problem, and so you can produce a relatively mundane cartoon in 2D, especially if you use limited animation. For examples, see King of the Hill, Rocket Power, or As Told by Ginger.

It should be noted that mixing live-action and CGI is often cheaper than making a truly hand-drawn 2D cartoon—you do have to pay the artists, and hand-drawn cartoons of good quality require a lot of panels. CGI special effects allow the filmmaker to use both real humans and elaborate special effects. Stop-motion animation is cheaper still but doesn't blend in as well. Old fashioned special effects are cheaper than that, but they can't do everything CGI can; even when they can, CGI is often less destructive or dangerous.

Here are some good ways to justify using animation:
 * The entire cast is animals. Funny Animals demand it. Other varieties depend on how many animals you want in the cast and if you have a humane society demanding that any animals in your film survive it unharmed.
 * There is at least one Talking Animal. Again, if it's a Funny Animal, it's all but mandatory.
 * Sometimes, the choice between a talking animal and a non-talking animal that provides narration depends on whether there's a humane society observing your work. If there is a stigma against injuring animals in films, it limits the amount of stuntwork a live-action animal can do, and thus what you can film them doing.
 * There is a lot of cartoon violence, or violence that causes extensive damage. Insurance companies prefer CGI violence to the real thing.
 * Live-action actors are a different pool from voice actors, and the cost of live-action actors is significantly higher than that of voice actors; or you want a phenotype that you cannot get in Real Life by conventional means. These together cover the more mundane varieties of Anime, especially anime with a significant number of characters that do not look Japanese. The second point covers works with Serkis Folk (remember, CGI is animation regardless of where it is).
 * There are children in a dangerous production, or many children in a TV series. Child-labor laws and safety regulations mean that it's nearly impossible to get underage stuntmen. Also, past a certain point, voice actors are cheaper and easier to use than real children. This goes double if live-action actors are free agents.
 * You want to use an actor in a role that suits their personality but not their physical type.
 * You are using a fantasy setting, especially if it has native life or magic.
 * Toon Physics are being used.
 * If your country has an Animation Age Ghetto, then making a work completely animated, especially in 2D, is often a good way to remind scriptwriters, actors, ratings boards, and audiences that you are aiming for inside the ghetto.
 * If your country doesn't have an Animation Age Ghetto, however, you can use full animation for works that aren't X-rated but are meant for adult audiences.
 * You are already animating something else in the film. If you've already brought in the CGI company, then you might as well use enough CGI to cover all the trickiest bases or to make it a good marketing point. You can also add in points for Rule of Cool that wouldn't be worth it if there were no other reasons to use animation. And if a sufficiently high percentage of your work is animated, leaving live-action elements in will either be more expensive than full animation or cause visual clashes.

A specific application of the Law of Conservation of Detail. An animation trope, obviously. However please note that as the cost of computer animation tools and the minimum skill required to use them both drop, this trope may begin slide into discredited territory and beyond.