Why do English recipes of “sauce marchand de vin” call for Worcestershire sauce? The “sauce marchand de vin” is a French red wine thick sauce typically served with meat. Its recipe in my French cookbooks call for two main ingredients: red wine and brown stock. It also uses shallots, butter, flour and black pepper, but I understand the two ingredients cited previously are the main ones.

However, many recipes I can find online (here and there, for example) on English-speaking websites add Worcestershire sauce. Not all of them do, but I still wonder: what purpose does this extra Worcestershire sauce add? I'm not too familiar with it, but if I understand it might bring some spiciness (already somewhat covered by black pepper).

So, what do you think it brings to the recipe and overall taste?
Worcestershire Sauce is added where the recipe wants a fast way to develop or add savoury richness, umami. 

It's often used where umami would develop over time with slow careful cooking (and heavy bottomed pans). Adding this extra ingredient is a good cheat where you just want that kick without the wait.