To really understand DOF you must take a camera (one where you can change the aperture) and take a perspective oriented image.
If you take pictures with apertures ranging from 1.8 (if you have a good lens)
you’ll see you have a very narrow field that has focus.
The field before and the field behind the focussed area will be more more and more blurry the farher it is.
If you take a picture with an aperture of say 16 you’ll have a huge field that has the focus. So a lot of sharpness.
you could say “Why? Why not make everything sharp?”
Well, the lens is built that way. You can only take a picture as long as there is light.
In bad lighting conditions a photographer will have to open up his lens almost completely to be able to take a picture at an acceptable speed.
With extensive light, the photographer will have to keep his lens almost closed so his pictures won’t be overexposed.
In a CG image you’ll want DOF effects to have a photorealistic image or to dramatically enhance the image.