Hola Valerie

The imperfect progressive and the Imperfect have a similar use. They can both be used to express that "something was happening" in the past, in a descriptive way. For example:

"Los chicos jugaban en el parque" or "Los chicos estaban jugando en el parque"

Both options refer an ongoing action in the past. The progressive tense gives a bit more sense of an action in progress.

However, we use the Imperfect to express what someone "used to do" in the past, for example:

"Cuando era pequeña, jugaba mucho al escondite." (When I was little, I used to play hide and seek a lot.) 

Here, the past progressive would NOT be an alternative.

I hope this helps.

Saludos cordiales

Inma