There are quite a few species which needs poison as their prey are larger in size. Like the black widow, it's small but it needs the poison to protect itself. But strangely its also web weaving. Singapore have lots of species but most are not poisonous and their poison is not strong enough to kill humans or animals.
How I know all these? Cos I love spiders, I have lots of books abt them. Had them as a hobby once but GF scared of them, so gave them away.