What is the use of Brahmins in settlements?

They can be used to carry caravans around and can be killed for their meat as said here. They can also be used to see dialogue actions and seemingly help boost productivity as well as said here:

"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." (This quote plus the link comes from @SlashTag)

Brahmin is way more important than you thought. As we know that the best way to get rich is selling purified water. But the problem of selling water is purified water is heavy. So here is another way to get rich: 1. Feed as many as possible Brahmins 2. Brahmin will produce fertilizer (in your workshop) 3. Use fertilizer (and plastic) to make Jet 4. Selling Jet

Jet's weight is only 0.1 and it's expensive than water.

Better to use these 2 ways together, so that you can use purified water to exchange plastic.

First of all I'm pretty sure Brahmin don't just spawn in your settlements, there is an NPC walking around selling Brahmin for 100 caps, and this person can usually be found anywhere between Sanctuary, Starlight Drive-in and Abernathy Farm.

I bought a brahmin off her and had to choose which settlement to send it to. I chose Starlight Drive-in and sure enough my Brahmin was there when I showed up so I built it a feeding tub.

I don't believe it's wise to buy brahmin for slaughtering purposes, I know they provide meat and leather but it isn't worth 100 caps a head. I do believe that brahmins produce fertilizer (shows up in 'junk' in your workbench).

Also, contrary to what many believe, you do not need brahmin to establish supply lines, I think you only need Local Leader perk (level 1) to do that, and supply line brahmin spawn automatically.

I believe they also give your settlements Brahmin meat in the Workshop inventory.

After enough settlers and food troughs they will start coming in and killing them is only hurting you. Get as many as you can for they add to happiness and productivity. As well as they give you fertilizer which you can use to make ammo and jet which can be sold to make a load of caps.

The Brahmin produce fertilizer.

I believe only give you fertilizer and count as a settler , and you can only have 10+ what every you cha is in settler .