what are the benefits of partitioning an external hard drive?

None really, unless you somehow want to install an operating system on one of the partitions and use the other just as a data storage.

When i buy a new HDD, i will partition it, want to know why?
To have an extra partition to use for other systems, but the oher part will go for being a brother for my current.
Partitioning is only useful if you are sure of how mcuh of the HDD you want to take off.
If you then make a too little one and its still installing a system, then its not good at all, because you cant make it bigger, only delete it or make a new one.