Hey bro,
The rear delts are only a small muscle group but are vital for two reasons; they fill out the shoulder proportion and make you look bigger over the whole upper body (guys with small rear delts usually look round shouldered and often complain their shoulders are small) as the rear delts are needed for the look of shoulder size, and; the extra strength of the rear delts helps to hold the shoulder joint intact as heavy exercises are performed.
It is amazing the number of people who do heavy benches and presses without strengthening the rear of the shoulder. This leads to injuries from shoulder tendinitis to torn pecs.
I see that you want to increase your bench press; you will be surprised at the big improvements in your PB for bench press in just a few weeks of heavy rear delt work.
To answer your question on rear delt single dumbell flyes; it is really the only exercise worth doing, this exercise lets you perform at a great range of motion than using two arms as your back do not cruch together and limit your weight and range of motion; you can also focus more on exercise.
Here is how to do it:
With the dumbell in one hand, bend over at the waist and place the other elbow on a bench.
Keep your back straight and parallel to the floor, and your knees slightly bent, lift the dumbell from the bottom to above your shoulder height. Use a little abit of swing as it help you lift more weight (not too much swing though)
That is how I broke my PB for benching from 290 to 310.
It really helps with looking bigger particularly if you have over developed traps that can make you look bunched up.
Cheers Cobin
Mad Mongolian