why does ja appear in the middle of sentences sometimes when it doesn't seem to be necesssary

Ja seems to crop up in the middle of sentences quite often but I don't understand what is function is or how to use it when I make up my own sentences.
For example from Level 2 12.2
Das können wir ja noch besprechen…
Du siehst ja so gedanken verloren aus.
 Du verschwendest ja keine Zeit.

Can someone explain how to use it? Is it for emphasis?

You are right it is used for emphasis. Apart from 'ja' meaning yes it can also be a filling word amplifying a certain notion. As English doesn't use 'yes' as a filling word it might be a confusing concept.  You have noticed that it is positioned differently as it becomes an adverb in your examples listed. 

Gruss

Paul

Thank you for the explanation, it's good to know I was more or less on the right track. I hadn't thought about ja as an adverb.
I tried putting my examples into Google translate with and without ja and it didn't change the sense but for : Das ist  interessant /Das ist ja interessant  It changed the translation from that is interesting to that is very interesting.

Hi Sfpugh,

That is very bizar. You never know what google translate is going to do. I don't trust it too much and rely on other online dictionaries like LEO.
https://dict.leo.org/englisch-deutsch/

Gruss

Paul

Yes, google translate can be a bit weird. :)
Thanks for the Leo suggestion.
I have been using Langenscheidt which seems pretty good except for the flaky search box and also Beolingus.
I was impressed that LEO had "verpeilt" which the others don't have. I have only heard of this word as youtube Easy German  did a video on it.