You can purchase plan options so that, when "abroad", calls to your originating country, or to numbers on your same operator, or landlines, or data traffic from abroad, or email services specifically, or whatever, will cost you less.

Often, such options will only work when you're on the proper partner operator abroad.

For example, with 10 Euros/month I used to purchase a plan option that, for a limited number of minutes, would make my calls while abroad cost me as if I was in Italy - where my SIM is originally from -. Since I have a Vodafone SIM in Italy, that would only work when using Vodafone in France, for example.

Unfortunately, at least here, such plan options keep changing name and features. It's just a mess to know what is available and whether you actually need it or not.