why do mosquitoes and leeches suck blood?

Female mosquitos use blood in the development of their eggs.

Leeches are just hungry.

For mosquitoes blood is required for reproductive success.

For the leech blood is food, fir it's survival.

well mosquitos suck the blood and replace it with protazoa which can then cause malaria, unless you have sickle cell anemia or the trait this way you become immmune to malaria but have sickle cell, leeches suck the 'bad' blood that is decompsing going brown basically, they ont suck fresh blood which is a common miscomseption

Nature cheated on the mosquito by making a probosis instead of a mouth, thereby having the sucker tank up blood instead of a big mac. As for the leeches, how cruel to have a micro-mouth and unable to swaller a triscuit.

Adult mosquitos actually eat the nectar of flowers. But mosquito babies need protein, not sugar, to grow. So their mothers feed on blood. Bloodsucking mosquito moms find you by sensing your body heat and breath. Then, with their proboscis, they drill a hole through your skin, into a capillary. Their saliva keeps the blood from clotting while they drink.

As we derive our energy from the food we eat such as plants and animals, leeches and mosquitoes find that drinking blood is the way they gain the energy to respire. It is simply their food source. Similarly, tics, fleas and other vermin that live in fur and hair also suck blood.

I have seen this technique being used on the throat of a cancer patient. But it didn't cure the disease. The method is adopted from very old habits of people when antibiotics were not available. However I feel that this technique merits investigation in laboratores.

Why do humans eat almost raw steaks where the blood oozes out of them and the meat in the center is still cold? Tastes good.

Because that is how they eat. Blood gives them thier nutrients.

Because they evolved to feed this way.