What do we get from animal foods that cannot be supplied by fruits and vegetation, including nuts and seeds?
Here are a few:
With the exception of B12 you can theoretically synthesize or convert plant derived compounds into these nutrients, but you can't get them directly from plant foods.
I guess they wouldn't count, but creatine, conjugated linoleic acid, carnosine, carnitine, and the long chain omega-3 fatty acids are either found in tiny amounts in non-meat and/or are only in a select few non-animal foods.
Aside from all the great sciencey answers above, I would add, no...it's not "necessary" just the same as its not "necessary" to eat healthy to survive. I know plenty of folks that eat lots of processed shit on a daily basis and seem to survive just fine. Who knows, they may even live longer than me....but to "thrive" I think it's essential to get nutrients from healthy animal sources.
I haven't seen anyone say carnitine, creatine, and carnosine yet. Not necessary per se but having an optimally functioning brain isn't necessary per se.
I'm for thriving as well as surviving.
Long chain PUFAs: Less than 1% of linolenic acid is converted to the active DHA, same for linoleic acid to AA.
Vitamin B12: Only found in animal foods, some plant foods are tested and say they have B12 but this is analogues that are similar chemicals close enough to pass the test but are not bio-available.
Vitamin D3: Some mushrooms have vitamin D2 but this is very ineffective compared to D3, found in oily fish , cod liver oil, and pastured lard.
Vitamin K2: K1 is found in leafy green vegetables and is for clotting, but K2 is found in animal foods such as pastured fats and dairy. Natto has K2 but it the MK-7 form which is poorly converted to the true MK-4 form. A tiny bit of K1 is also converted to K2 MK-4 too, this is where is comes from in pastured fats (grain fed doesn't have K2).
Vitamin A: 45% of the population can't convert beta-carotene AT ALL into retinol, the active form found in liver, eggs, etc. And most who can convert only convert a small amount.
There are many nutrients that essential can be made from the plant forms but humans don't do this well efficiently unlike herbivores which can. This shows we're adapted to letting other animals convert and concentrate our vitamins for us.
If you were to consume no other animal food, the best would be fermented cod liver oil as it contains all of the above except for K2. K2 can be supplied from butter oil or supplement such as Thorne's K2 MK-4 drops.
A lack of these nutrients is the cause of most problems on vegan diets. B12 deficiency causes nervous problems, and lack of retinol / D3 / K2 results in brittle bones, cancer, etc.
Pleasure! Have you ever had some crispy bacon? A nice grilled rib eye? A juicy burger?
Aside from that, I think animal foods provide a good source of complete protein, healthy fats and loads of nutrients that is easy to digest, has a low likelihood of causing allergic reactions, lacks antinutrients, etc. Even if you can replicate that with plants, it requires very careful planning to make sure you get everything you need and careful preparation to neutralize the bad stuff and pre-process the fiber as much as possible. Not worth the effort as far as I'm concerned - especially since there is no real benefit to avoiding meat.
Also, when you raise solely vegetables/fruits, you only have part of an ecosystem. It requires artificial fertilizer (and that is not as good as the real stuff). It destroys top soil and is not sustainable. Ecosystems need plants and animals. Read the Vegetarian Myth.
Also, as Jeff just said EPA/DHA only come from animal sources, but in theory your body can convert ALA from plant sources to DHA/EPA. Some argue that conversion is not efficient, but I think that depends on how much DHA/EPA you're getting. Also, complete proteins, but of course if you mix plant sources (e.g., rice and beans) you can get complete protein. Potato also is complete protein (though in low amounts).
Denise Minger has a good page for vegans that hints at the things they might be missing in their diets:
Most vegetables are shitty sources of nutrients due to the cellulose/oxalates/phytic acid, but they're good for detoxing the body. Vegetables have their place.
If your concern is high nutrition, animal foods are the way to go. No two ways about it.