What is the purpose of these labels on an aircraft's wings?
Closer than you may think: notice that these panels all have fasteners visible-- these panels are removed frequently for engine work.  The labels are there so that the maintenance crews can both more easily reattach the panels, and so when they're inspected (again as part of maintenance) you're can quickly confirm that everything is in the right place.

When you have a plane in parts, all the panels start to look very similar lying on a workbench... Having the labels makes things more efficient and safer.