Being sealed for life has to be a misnomer.
If you get a failure, major or partial, you still have to be able to fill them to the correct level.
As well as have clean oil passing through the device.
Filters have a service life, so they have to be accessed.
It can be a problem when owners actually check the oil level, see it as low(cold) and then put more in.
Or check it without functioning the box through the full range, to get an accurate level, and drive it underfilled.
It's an effort to remove operator error.