Why and how would I land straight-in from a circle-to-land approach?
The sentence before the one you bolded, with my bolding applied, says:


  When either the normal rate of descent or the runway alignment factor of 30 degrees (15 degrees for GPS IAPs) is exceeded, a straight-in minimum is not published and a circling minimum applies.


So you can have a "circling approach" that is lined up straight in to a runway, but with only circling mins, not straight-in minimums, published.  This could be driven by the need for a high FAF altitude, high enough that you can't get from that FAF at that altitude down to the runway, with a descent rate that's within the TERPS criteria.  So they build an approach that assumes you maneuver to lose the excess altitude, and give you circling mins so that you can.

However -- and this is what the exception that you bolded in the question is about -- if you're in, let's say a STOL airplane, you may very well be able to lose the excess altitude and be established on a normal visual glidepath to the runway with ample time to make a "normal approach and landing" without any S-turns or other maneuvering required.  And in that case you're absolutely allowed to fly the approach straight in.  The guy behind you in a Lear Jet may not be able to do the same thing that you did, and the TERPS descent profiles criteria are written to guarantee that if he has straight-in mins, he CAN get from the FAF at its crossing altitude to the runway without having to use what would be, for him, a dangerously high descent rate (angle).  But in the STOL aircraft, a steep descent is perfectly normal, so you can go ahead.  The Lear shouldn't try to mimic the Maule's descent profile, but that doesn't mean that the Maule has to mimic the Lear's descent profile.