The following was observed in the US --- it may be an American phenomenon.
I have recently observed in my restaurant visits that more often that not, if I order a burger, sandwich, or a hotdog, I am often served, in addition to the dish and side-dish, a single quarter-slice of a long pickle. Why is this done? The pickle is obviously much too large to put on my sandwich or burger --- slices of pickles would be better for this purpose. I also can't squeeze the juice onto the dish. All I can do with the pickle is essentially eat it raw and whole. This strikes me as curious. I don't see why it is included.
Intuitively, it seems to me that this must be a traditional custom. What is the origin of this custom? Why do restaurants do this so frequently?