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.
Actually, with the exception of a small minority, all spiders are poisonous (venomous actually, since they inject the venom by biting, while poisonous denotes having to ingest it, although of course anyone would understand "poisonous"), and all spiders bite their prey to kill them; the prey is not killed by injury from being bitten or from the web in web-spinning species.
But the venom of most spiders is completely harmless to humans. Most of the common ones we see cannot penetrate an adult's skin with their small fangs; a number of species can give a very painful but usually harmless bite, only a few can actually cause serious illness or death (I'm not familiar with many Singapore spiders but I don't think we have deadly ones here).