When cooking, is it necessary to devein shrimp?

It is done because
A: It looks better and
B:The vein is actually the intestines and has the fecal matter in it.

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Thanks – that’s very clear.

Not necessary, but it’s a lot less gross. It doesn’t change the taste, but it’s poop, like @Dog said. It puts squeamish people, like me, at ease.

the vein is poopy ,it taints the meat if left in whilst cooking

Not 100% necessary, but gross if left in. The texture also leaves something to be desired – it’s a little gritty.