[baruch]

It reduces context switches in the kernel which are more expensive and reduces the memory resources needed for kernel threads which are larger.

There are also some hidden gains in terms of TLB caches and other costs of kernel threads switching.

An additional advantage is that between user-space-threads you have fewer locking problem since they implicitly lock out each other between context switch points so you only need locks when you need to protect an area across several context switch points.