For me personally, I only do this if I have more storage space than needed. But, splitting them onto separate partitions is mostly about risk management and flexibility so reasons may vary:
- /home: Keeping it separate means you can reinstall or even switch distros without wiping your personal data and configs. It’s one of the simplest ways to make system rebuilds less painful.
- /var: Useful if you run services that write a lot of logs or cache data. It prevents logs or a runaway process from filling up root and crashing the system.
- /tmp: Isolating temporary files keeps them from consuming critical space elsewhere. Some people even mount it as tmpfs in RAM for speed.
The downside is managing disk space across multiple partitions. If you misjudge sizing and /var fills up while /home has plenty free, you’ll have issues. That’s why lots of us just keep it all on / (root) unless there is a specific use case.
So in practice, it’s a trade-off: simplicity versus flexibility and available space.