Why is trading good for both people or groups involved?

Both sides get something out of it. If people are trading it is likely they are trading for something they don't have direct access to and therefore need to trade something for it so someone else can give it to them. If the trade is fair, both sides are happy and satisfied with the item(s) they received.

2018 means 2018 ce or common era

there have been other eras too including bce, which included way more years.