Why are red blood cell transfusions commonly used to treat some patients with anemia?

Replacement. Anemia is basically a lower than normal number of red blood cells. If it is bad enough, it may be life-threatening because red cells carry oxygen to tissues. A transfusion fixes (at least temporarily) the problem by "filling the tank". Transfusions carry their own risks and are not treated lightly by doctors!
Safety. If anemia is severe, whatever the cause, blood transfusion may be necessary and even life saving. Whenever possible, the underlying cause of the anemia should be identified and corrected, but if the anemia cannot be reversed quickly (like massive bleeding for example) transfusion may be the only way for the patient to survive.

Related Questions

Test CBC readings some red blood cell are low and mono is high, anemia or not, what to do?

Insufficient info. It would help to have the actual value and why the tests were done? All laboratory results need to be interpreted in the clinical context and the doctor who ordered the tests is usually in the best position to do that.