Whenever I see pictures of trucks made by Volvo, Scania or Daimler.
I don't see those mirrors in most trucks made by Indian Commercial Vehicle makers like Tata Motors.
They provide the driver with coverage of two significant blind spots. Especially in cities, there are numerous accidents with cyclists, pedestrians or even small cars being in that blind spot and being hit.
These mirrors allow the driver to cover those blind spots and be sure no one is there before he begins a manoeuvre.
Safety legislation in Europe may be the reason they are fitted on these ones, but not on Indian made ones.