Hola Nick,

In this lesson we are only dealing with the conjugation of the tense, therefore there is no emphasis on how/when we use it. We want the students to focus on the form only. But we do have a different lesson explaining when to use it, and we compare it with the other "was doing" which is El imperfecto progresivo, "estaba haciendo". 

As previously mentioned to Papi in a previous question, the preterite tense (progressive or non-progressive) doesn't always come with a time phrase. Sometimes that time phrase is known as it was mentioned before in the conversation, or simply, you don't add a time phrase. When using a preterite tense, the speaker is seeing that action in the past as a "completed action". If you have a look at these two lessons, everything will be clearer:

1. Lesson on difference between El Indefinido progresivo and El Imperfecto progresivo

 

2. Lesson on difference between El Indefinido and El Imperfecto (general use, with no time frames)

 

I hope you see it with more clarity after seen these two lessons.

Un saludo

Inma