Hi Alisa This expression is used to tell someone to do something while he is doing a similar activity. For example: 'I’m just going to clean my boots.’ ‘Well, you can clean mine too while you’re at it.’ You can find a few more examples here: https://www.merriam-webster.com/dictionary/while%20(one's)%20at%20it You might want to read those while you're at it. 😉 Hope it helps.

The implication is often that doing the extra thing would be less work if you are already doing the first thing. You can see that in most of the examples you have been given, except the McD's cleaning one.

Your examples help me a loooot!! Really clear!! Thank you all!!

It is used to refer to something additional being done related to the first thing. For example, imagine that you work at a McDonalds. One of your tasks for the day would be to sweep the floors. Your boss might see you sweeping the floors and say, "can you wipe the counters while you are at it?". He is adding another related task for you to do since you're already cleaning (already "at it"). Another example: you could be going to the grocery store to buy cookies and your friend sees that. So your friend asks, "can you buy bread too while you're at it?" You're already buying cookies at the store so buying something else there is related and you can do it simultaneously.

There is indeed such an expression. "Will you go out and grab the mail? And while you're at it, you might as well take out the trash too."