Usually corruption. If you ever need to reinstall and don't want to reinstall everything, you can just install or fix that one partition.
Separating your /var and /tmp files is always a plus too as those change in size all the time, with logs and temp files being used/moved around. Lets say if something happens so your logs fill up your one partition, your system won't boot then. With its own partition, it will only fill that up, making it easier to fix.
Also making a separate home directory is also good, so if you do ever reinstall, you can keep your home directory if uncorrupted and you don't have to go back, reinstall or setup those home directories again.
Usually you want to make your / and /usr separate as programs are usually installed on /usr which require more space, its just a good idea to have it separate from the rest as well.

If your going to be installing for learning though, and maybe not going into X so much, you should easily get by with a small swap partition though, that shouldn't be any problem. But if you want to learn Linux, create multiple partitions just to get your hands dirty and see how it all works, cause any server you ever encounter will most likely have separate partitions. That in the long run will help you learn how linux works as well.