Why does a subsonic nacelle inlet have a throat before the diverging section?
The throat is a byproduct of trying to prevent flow separation at slow airspeeds or high angles of attack.

Below you can see a rudimentary sketch of the intake streamlines for different airflow conditions. As you can see in the first case, the air needs to curve around the tip of the lip at low airspeeds. Flow will separate, if the curvature is too strong. Flow separation can lead to all kinds of problems for the engine. In order to prevent flow separation, a thick and rounded lip is used. Superimposing such a rounded lip on the divergent geometry of the inlet results in a throat as you described.

Image by K. Aainsqatsi - self-made, CC BY-SA 3.0, https://en.wikipedia.org/w/index.php?curid=11098086