Well it does actually ruin the nutritional value of the meat. The more you cook it the less water it retains which means the vitamins (typically the Vit Bs) will leech out.

In addition the proteins will denature which in the early range (medium or so) is a good thing, but when you start getting into the well done range they can reform into structures that have less bio availability, and (depending on the cooking type) can start forming compounds that are potentially carcinogenic.

Also the more cooked the meat is, the more bioavailable iron and zinc become, which is technically good unless you're someone with excessive intake of those minerals.

Oh and of course, by the time the steak is well done the vast majority of fat has been cooked out, which is technically great if you're trying to lose weight, but fats are an important part of our diet (especially saturated fats for men). The level of bio-availability of lipids in a well done steak is extremely lower in comparison to something around the Medium to Medium-Rare range.

Edit: Why am I getting downvoted for stating what happens the longer you cook meat? People keep bashing the "Meatlitists" but it seems like there are just as many "I eat what I want" people who can't accept a dissenting opinion, or apparently fact.

Edit 2: https://realdoctorstu.com/2012/04/30/rare-medium-or-well-done-the-science-of-a-perfect-steak/ -His citations are to real published studies.