It's very interesting that you use FWHM - I've seen that exact concept come up a lot in several physics courses, but always about oscillations or electrical filters (Q-factor basically). Funny how stuff like that comes up in seemingly disparate fields. But minimizing the FWHM for a telescope would be analogous to increasing the Q-factor for a RC filter, which is optimal and gets a "sharper" peak - which for a telescope would put the object in sharper focus. So I suppose it makes sense!