Either internal or external trade routes, potentially. If we connected our capital to the coast at the moment, we have a discontinuity in our potential range for trade routes because of the fogged tile 2W of clams. We accepted that we would be able to bust that fog soon enough. Similarly, if we see that SW is a peninsula and don't see enough tiles for the hypothetical future trade routes, when/if we make an AI contact up the western coast (and have Writing and Sailing) we may find ourselves unnecessarily short of trade routes. It might not be so convenient to solve that issue at the time (barbs, say). The trade-off is a turn or two of scouting the peninsula vs scouting up the western coast faster, for the benefit of not needing to come back and finish the job.
frogdude pointed out that the river heading north might do some of this job - if it reaches the western coast, then we don't need to defog all the coast tiles as well.
One possible scenario is that we settle the coastal gems site, build an overland road to it (lacking Sailing at the time), and settle our third city on the coast SE of cows at about the time we get Sailing (say, for city two to try for GLH). Now we'd want to have defogged the coastal route from city two to city three to get the free +1

/turn before we've had time to build a road to city three. Similar cases might exist if there's an AI up the western coast and we get Sailing and Writing.