Strictly technically speaking, not much, however having working Linux Open Source drivers extends the life of the product using it well beyond the point Microsoft, and/or the hardware manufacturers, declare it obsolete then stop any further development for that chip. There's a huge number of old systems out there that are supported by mainline Linux but couldn't run anything newer than Windows XP or 7, and they aren't just laptops. If you have put some money into a piece of hardware that still works perfectly and are forced to ditch it because either its manufacturer or Microsoft suddenly say "sorry, no more drivers, but here's an offer for the new model", that definitely sucks.
