It seems to me that at least two very different products are being discussed in this thread, and that all are usually described as "window film":
1. A thin film that is applied directly to the window glass. It is cut and measured carefully, and applied to the inside of a window with a liquid (sometimes water, sometimes water and a surfactant). This type of film is often familiar to people as the type used to tint automobile windows. It can have a tint, be clear, or even be "mirrored". It usually offers significant UV protection (my primary reason to use it) Some varieties reflect heat and sun "out" (used in sunny, desert climates). Some reflect heat "in" to the house (cold, overcast climates). These are not for the purpose of slowing or stopping drafts from leaky window seals. The do sometimes help with convection drafts because of temperature changes in the clear part of the window.
These can be very fiddly to apply well. It is often applied with a squegee (not even sure how to spell that!). It is easy to trap air bubbles between the film and the glass, which really does not look good at all. Once you put these on, you leave them on for many years. You can still open and close your window. No functionality is lost. There used to be some controversy about using them on double pane glass. Some thought it increased the chance of cracking of the inner pane. I have used these on home windows in two houses with no problems, but this may be luck related. Sometimes the installer leaves a quarter inch of glass uncovered at the edges. I have been warned that application can void window manufacturer's warranty for some companies.
2. A thin (or thick) film that is applied to the frame of the window to create an insulating air gap between the the inside of the room and the glass. The films that "tighten up" when exposed to the heat of a blow dryer become almost invisible. These are MUCH less ertxpensive, in my experience, that the technique in #1 above, but are for a different purpose. They work great for stopping leaks and reducing. They keep you from opening the window. I used these in apartments and older homes I lived in. They worked great and made the space much more comfortable and lowered heating bills. They caused no permanent affect on the window....easy to remove. Mine were not reusable. Their advantages remind me of the benefits of old fashioned "storm windows" without the cost, hassle and storage space.
And there are many variations on these themes. These two are both "inside" options.