Don't get me wrong, I love it, it's fun but once you have all the bean rewards, whats the endgame for the farm? Ports has the bones and rocktail soups but everything you can harvest from the animals seems really.. cheap? Except for dragon poop maybe which is nice but is that it?
By having the shiny perks, you can get: - increased chance of ghostly essence drops (money or improved use for prayer xp) - not using up resources in certain circumstances - increased dmg against spiders and arraxor - extra chance at effigy
I will admit that some of the perks are rather niche.
The other aspect is that you can have a self sustaining source of Farming XP. Tree seed prices have dropped since release. Thus making tree runs more economical.
Plus, you can have an increased number of cute and cuddly pets locked away. Or just have that animal cruelty potential and feed them to the baby troll on the farm.