I cannot figure out why we would exchange gold for gold, silver for silver, date for date etc.

In the same measure or weight even if the quality differ, if not it's becoming Riba (as mentioned in one module)?

In the past the people used to engage in barter trade where sometimes they would exchange dates of a lower quality for dates of a higher quality whereby one would give more of the dates of the lesser quality and get a lesser quantity of a higher quality of dates in return. There could be various  reasons for this; maybe  one  would  have  wanted dates, silver or gold  of  a  certain quality for resale or personal use and had  no money  to acquire that so they would resort to barter trade. In the past money was there but obviously it was not universally available like today, and barter trade played a role. In the present  day  age  barter trade occurs  in  some countries and some countries which had  economic  sanctions imposed upon them had  to survive through barter trade. Therefore these principles can still be applicable in today's world.

And Allah knows best