Why use an OTP if you already have a secure channel
First, let's re-iterate that there is no true one-time pad in modern cryptography or any other information system - it's only a hypothetical thinking reference. 

The reason to use OTP in this case, would be to maximize the utility of the rare window where there would be a secure channel. 

Let's imagine a world without internet and public-key cryptography cannot be carried out efficiently by any instrument available to the people and the enemy, the secure channel is a scarce resource, so a huge OTP stream is exchanged here so that it may be used later over the radio, possibly chunked according to the size of messages. 

This way, the time to exchange the actual (encrypted) message can be freely chosen.