It's mostly about removing checks for rare occurrences: * catching of NULL pointer references (ie. transforming SEGV into catchable userspace exception) * test

It's mostly about removing checks for rare occurrences: * catching of NULL pointer references (ie. transforming SEGV into catchable userspace exception) * testing whether some in memory data have been changed/accessed (ie. mprotect(), wait for SEGV, set flag, un-mprotect(), return) * paging in userspace (eg. across network) All that seems obscure (and is mainly useful for virtual machines and such), but it is not so rare to find process that actually does this (usually because of some library, or because it is some kind of virtual machine)