To each their own. Here are the reasons I can think of at the moment that I eat mainly suet and low-heated tallow for my fats (along with some grain-finished marrow, fermented CLO and fatty meats):
1. I prefer to eat fats from only mostly-pasture-fed or wild sources (the only conventional supermarket foods I buy are lean steak for beef jerky and mineral water). I find that for me, it is particularly important to get the fats and ground meats from good sources--I feel significantly better when I do--whereas it makes less difference on how I feel with the lean meats (and the pasture-fed lean meats are exhorbitantly expensive).
2. the grass-finished suet in my local healthfood store is cheap (99 cents to $1.99 per lb)
3. low-heated-tallow is my favorite-tasting fat (heating it at standard higher levels makes it taste burnt to me and eating high-heated tallow causes me to burp quite a bit); better even than raw or melted marrow (raw suet is rather dry and doesn't taste very good, so I chop it more finely and mix it in more thoroughly with the meat when I eat it raw)--which surprised me
4. the only marrow available in local stores is grain-finished (which may be part of the reason why it doesn't taste as good as melted grass-finished suet) and when you factor in the weight of the bones it costs more per lb for the grain-finished marrow fat than for the grass-finished suet
5. the market that sells the grass-finished suet is much closer to me than the one supermarket that sells decent grain-finished marrow
6. I only have a very tiny fridge with micro-mini freezer in a tiny, cramped apartment and I may be moving soon, so for the time being I'm not going to be buying a chest freezer so I can buy from Slankers or a local farm
7. I don't do well on extra virgin coconut oil and it's much more expensive
8. the only other alternative in my local markets that comes anywhere close in terms of quality to the suet is local pasture-fed, cultured, pasteurized butter. I don't do as well on it as the suet and the butter is far more expensive. There's simply no comparison.
9. I need to add fat to the grass-finished ground beef I buy, the fatty cuts of grass-finished meat are much more expensive than the grass-finished ground beef, and even some of the fattier cuts of grass-finished beef are still not fatty enough to meet my needs without adding fat
So there's really nothing remotely as good an option as suet in my situation. If you were in my situation, wouldn't you do the same thing?