Is there any rweason why we need poisonous snakes in the world the non poisonous snakes can kill the mice and

rats and other vermin

Poison to us may also be a cure for other things. Scientists extract the venom from every species of the world in hopes to crack the biological code for terminal diseases. I truly believe every plant, animal and insect has its own special purpose for our survival. We really cant afford for anything to be extinct. Thats why we have crazy activists out there trying to save every dying specie. They believe there dna and the chemicals they naturally produce is vital in our circle of life.

Some snakes like the yellow rat snake and the eastern pine snake, are big enough to kill rats without poison. But not all snakes can overcome prey by size alone. In desert environments, concealment is much harder so a snake can't become as big without frightening prey away before it can get it. Also, some snakes have to hibernate and that's not easy for the big ones. A smaller snake like a copperhead or a rattler is small enough to hibernate easily and advance on its prey undetected. But to compenasate for relative smallness it had to evolve venom, to help overcome its prey. So poisonous rattlers, vipers etc are needed to control rodent populations in plains/semiarid areas.

What we might think we "need", and what the world has, over a long time, got sorted out as living communities, are very different things. You might as well be saying - "Hey, I don't like all these walls in my house - I'm going to remove them all to make one big room downstairs." Only after you take them out might you find out that some of the walls serve to hold up the house above them, with the result that your house begins collapsing around your ears. Casually making dramatic changes without understanding what you're doing is generally a recipe for disaster, which of course has never stopped any of us from doing it. We just blame the resulting disaster on our political opponents or our spouse or something. In other words, just because you don't know what 'purpose' something serves, don't assume it has no 'purpose'.

in this world each and every organism contributes to its balance, yeah poisonous snakes harm us but the earth needs each one of us from the tiniest bacteria to huge whales

Without the poisonous snakes we couldn't create some medicines with their venom

There's NO SUCH THING as a "poisonous" snake!
ALL are edible.
SOME are ***VENOMOUS***!!!