The eject button can usually close the tray as well. This makes it possible to put a disc onto the open tray, settle down into a chair, then start the film at one's leisure.
It can also happen that one switches off the dvd player at the end of a film, then forgets about the disc and leaves it inside the player for weeks, even months, not generally regarded as a great idea. Pressing the eject button on a remote while still in one's chair will leave the disc very visible, and make it far more likely to be removed in good time.
That's my take on it anyway....
_________________
On a mountain range
I'm Doctor Strange