For cutscenes and other special effects having more control over FOV in-game would be wonderful.
The idea behind the SEXP is the following, it has two variables:
1- Amount of degrees to increase FOV (if positive) or decrease FOV from current value
2- Time for FOV change transition in ms
So, I might use the current FOV SEXP to set it at 90 for the cutscene and use the FOV-change over time SEXP to have snap zoom in and out of the scene.
Similarly this FOV change SEXP could allow for distortion effects such as the first-person FTL effect seen in Battlestar Galactica. (lens FOV distortion before the jump flash)
While such an effect can be replicated with complicated scripting, this SEXP could allow for easier, smoother, FOV changes.