Past Simple vs. Past Progressive

I've been noticing in conversations that people often use past or present or future progressive where I would normally use past, present or future simple. I know some rules about interrupted actions that are always described by progressive tenses. But today during listening some podcast, the lecturer said

Initially I was planning to make this podcast...

So my question is why to use progressive tense here? Could we use simple here? I suppose there is some difference in meaning, but have no idea where to start digging.

Both Initially I was planning to make this podcast ... and Initially I planned to make this podcast ... are grammatical. The reason that the speaker uses the progressive aspect is because he or she recalls or conceives of the planning as being of some duration.

It is worth citing Michael Lewis in The English Verb (p42) here because what he says is fundamental to an understanding of the speaker's choice of tense or aspect in English.

We have already seen pairs of sentences both of which follow the rules of grammar as fact - in other words they are "correct" standard English. ... The differences (in meaning) are based on a choice made by the speaker at the moment the language was used. The importance of this idea is impossible to over-estimate. The speaker's understanding of the situation, intentions, and interpretation of the facts are central to the language the speaker uses.

Later Lewis reminds us that aspects of the verb, such as the progressive form:

... do not refer to real time but to psychological time - to the speaker's perception of the temporal quality of the event. ... The essential characteristic of (be) + ing forms is that the speaker uses (be) + ing if, at the moment of speaking, s(he) conceptualises the action as existing for a limited period of time.

This contrasts to the simple form, which:

... expresses the speaker's view of the event as a complete, unitary whole.

On this basis it is not helpful to think that 'during /period / time of planning is always followed by progressive' (OP's comment), but to use the progressive whenever you wish to convey the durative nature of an event.