basically, convenience and ease of maitenence, + it saves your OS (esp any flavour of Windoze) from corruption as quickly/easily.
Smaller partitions are easier to defrag if needed, as Bernard said it does help speed up booting a bit.
Also multi paritions with multi boots of 1 o/s or various o/s lets you configure them differently for differnt tasks, thereby causing LESS bloating of the system, (as Windows is notorious for being bloatware!)
the more bloated, the slower the system runs.
The 1 size fits all approach, often does not work in these days of modern computing and people using their machines for various things, from internet surfing and communications/business admin, to multimedia video & /or audio production to gaming!
Games especially need max resources to play well with modern graphics often pushing CPUS and graphic accelerator cards to the limit!
The less crap installed in Windoze and, services running, the better!
Hope that helps.