~17.13 is the common 70 FOV in many standard games like FPS based on a full frame sensor size
to a value you want to use when moving around the scene
For an example if we take 5.6 base and 32 extended we could have a nice natural depth you might be more accustomed to and would be useful when exploring areas where you want a little bit of depth, like an open vista.
roughly the aperture of an eye in terms of DoF
a lot of shallow depth of field good for object close ups
a common average many lenses and shooting conditions
a very natural lens setting, gives you a nice balance between slight depth and a sharper scene
From here an above, a great setting for the extended aperture to be set to for open scenes