By +ing vs. Ing

Question

Yesterday, the guy I was conversing with, told me that he is good at speaking English. Hearing it, I asked him right away "why are we wasting our time speaking in Hindi. (no offence to my mother tongue).

As I said it, I felt that I should have added "by" before using "ing" for example: "why are we wasting our time by speaking in Hindi.

I looked it up on Google and found out that the ing here is a gerund. But I don't know when to use gerund . I could not fathom the appropriate usage of it as the rules have been vaguely described.

So my question is :

A. Is there any difference between these two sentences?

B. Can I use ing without by preposition without affecting the meaning?

Answer 1

A) Yes, and I'll explain it in B).

B) It seems to me that using by in these phrases you wrote only emphasize the way you'll be doing the thing. For example:

Why are we wasting our time speaking in Hindi.
Straightforward phrase. No running around the context; you're saying it the way you see it.

Why are we wasting our time by speaking in Hindi.
You're explicitly telling your audience the way you'll be wasting time: by speaking in Hindi.

So you see, adding by in the phrase allows the receiver to explicitly see the way of how you'll be doing the previous action. But that's a bit redundant, if you ask me.