What is the use of cabins in singleplayer?

Cabins are used to convert your single player worlds into multiplayer worlds. So while you can create an entirely new farm with up-to three cabins from the get-go, if you wish to share your existing single player world to another player you can get Robin to build a cabin for them.

How do I host my farm?
Each player needs to have their own home on the farm, so the first thing you’re going to have to do is build each of your friends a cabin to live in. Don’t worry, they’re cheap! You can have up to three on your farm.

When your cabins are built, save, return to the title screen and open the co-op menu. Your file should appear in the host tab. ^Source

Since you can sleep in any cabin's bed, another minor benefit in single player would be to use cabins as a conveniently located bed close to the other exits of the farm.

The main house is closest to the upper right exit.
A cabin can be placed near the upper left exit.
A cabin can also be placed near the bottom exit.

Could come in handy when you are cutting it close to 2am and rushing back to your farm from one of the other entrances/exits.

In single player they could be used to get some cheap floor space for kegs/houseplants. They only cost 100g and 10 stone. But I don't think they would serve much purpose outside of that.