The simple fact that it's not 8, 10, 12, 16, 24, or 36 exposures, or the infinite spray and pray of digital, but one frame. You sit and wait and take far more time to make sure you get it right the first time. Letting the light hit the right moment, or even waiting for an entire day or two, to get the perfect photo.
Ansel Adams did this often, and one time he even stopped his car, pulled over to the side of the highway and jumped out, set up his 8x10 rig, and composed a photo as quickly as he could because the light, the reflection, and the setting was perfect in his own eyes. Needless to say, the negative was underexposed, but he managed to work with it in the end to make a final print. The full story is Here.
Well, with 35mm sure, this would be simple to just stop, lean out the window and take the photo, but you don't have the proper chance to really save a badly underexposed negative to make a truly awesome print with the amount of detail and clarity that you will with larger formats of film.
And forget trying that with a digital image, as the amount of noise that will become apparent trying to save it it, will entirely ruin it.
Well, I had the opportunity to try out 4x5 film, and I love the experience. I have a 4x5 Calumet CC400 Monorail view camera, which isn't the most ideal for field work, but is one of the best setups for 4x5 studio work, including still life and product photography.
Nonetheless, I have used it in the field, including a hand-held photo.
Learning the basics of 4x5 photography, and larger, can be daunting, and downright scary at times.
In the end, when you see your negative, and the final print, you realize that the time taken to take the photo is worth every moment, and entirely worth the expense and the experience.
Is it for everyone? No I don't believe it is. Digital shooters that fall under the "Pray and Spray" category should not use this format, as they won't be able to take a hundred photos for that single frame to stand out of the rest.
But those that are learning to take their time, or are feeling too limited in their current gear, such as digital, 35mm, or 120, can move up to 4x5 photography and learn the pleasures of shooting a larger format.
Of course, your results may vary, and not everyone will enjoy the larger formats of photography, as it is significantly slower than even using a 120 roll film camera, even if that 120 roll film camera has similar movements and functions as a 4x5 field camera or even a press camera.
Either way I enjoy it entirely.