The purpose of the outer door is as solid as the door itself and is well defined. An outer door is for safety, for security, and for protecting the home from the vagaries of the weather. Barring entry to all unwelcome elements, whether animate or inanimate, is an important function of a front door. Besides all this, its appearance is considered a beauty statement that the residents are making to the outside world.
As against all this, the interior doors are more for privacy than for security. With the inner doors, the job of partitioning the area is more important than the job of safeguarding. While no one will ever consider a front door unnecessary, everyone does not consider all interior doors necessary. For example, some people need a proper door between the kitchen and the dining room while others prefer to combine kitchen and dining space for easy access to cooked stuff. Here the door is not an absolute necessity but just defines some comfort parameters.
The door between drawing room and dining room is also optional. Those who have their kitchen separated from the dining room with a solid door might opt to have their drawing and dining rooms combined, without a door separating them. Some people separate the two spaces with beautiful draperies or bead curtains. Such screens are also a form of doors. However, saying that certain interior doors are not important does not mean that all interior doors are optional. Doors are mandatory for bedrooms and when they are there, they should be aesthetically satisfying with the right locking mechanism and should do the soundproofing job also satisfactorily.
Since the safety aspect is less important with an interior door, you can afford to give more importance to its visual appeal. Interior doors are ones that you can design yourself, giving free rein to your imagination. There are half grill interior doors that help in providing sufficient lighting to adjacent rooms. Privacy problem with half grill doors can be solved with heavy curtains, or an extra half door that can close the grills when necessary. A homeowner had designed his bedroom in such a way that the bathroom door was flush with the built in wardrobe of that side. The door of the bathroom and the doors of the wardrobe were made of identical material and given the same finish and had the same type of door knob, so that it was impossible even to make out that there was a bathroom to that side. The area just looked like a large three door wardrobe. Such unique door ideas can make a room very attractive.
However, whether it is interior doors or external doors or Interior design ideas, the door material should be such that it does not warp or twist even when atmosphere is heavy with moisture or dry in the summer heat. That is a non-negotiable rule while choosing doors.