I was working in my settlements and found my first Brahmin in Starlight Drive In. After some research on the internet, I found a way to keep it in one spot using the feeding bathtub, but still, WHY do I keep it? Nobody on the internet seems to have the real answer. Does it add food? Does it makes people happy?
According to this post on the Steam Community forums brahmin increase the productivity of the settlement.
The game doesn’t tell you what purpose the brahmins serve when they initially start spawning in your settlements. However, once you randomly start running into the wastelanders on the road who want to sell you one, you can find out in their dialogue options. They state that having a brahmin will increase the productivity of the settlement it is located in.
As well, as stated in the game’s help menu, increasing the happiness of a settlement results in an increase to its productivity, apart from preventing its revolt. So, my guess is that once you naturally increase a settlement’s happiness to a certain level (I don’t know the cut-off point), and therefore increase its productivity, a brahmin will spawn, as the concepts of “happiness,” “productivity,” and “brahmins” are interconnected.
I have only bought one brahmin so far, but I have six of them in my various settlements.