Rear entry sex is pleasing to both men and women because the angle of the man's erect penis means he will generally thrust against her G-spot, an area of tissue that can be exquisitely sensitive about two inches up her vagina on its lower surface as she lies on her abdomen. 

As you can see in the pictures below, rear entry sex positions do not necessarily imply that the woman is kneeling with the man standing behind her. She may be lying down or kneeling, and he may be kneeling or standing.

The next two pictures illustrate very clearly how exciting rear entry sex can be for both partners. The left hand picture is a classic illustration of rear entry sex: the man has the ability to thrust deep into his partner, to enjoy the sight of his partner, and to stimulate her as the couple choose. The right hand picture shows how the couple can alter the classic rear entry position so that they both gain greater enjoyment and sexual excitement: she bends forward, or clenches her buttocks, or moves her legs closer together, or lowers her head, or kneels with her buttocks raised - all of which are changes that make her vagina tighter or less tight, and alter the angle at which her man's penis strokes the inside of her vagina.

And there are more variations of sex in this position in similar vein:

Perhaps the most exciting sex position of all for the man is the one shown in photo immediately below. This allows him to get the closest possible body contact between him and his partner as they make love, to get the deepest penetration, and to feel the most powerful in his maleness as they enjoy sex.

Yet it would be wrong to think of this as a sex position that is mostly about male pleasure (rather as we might incorrectly think of the woman on top sex position as being chiefly about female pleasure). The four illustrations immediately below show a woman enjoying her sexual power, riding her man by moving back and forth along his penis as he remains still. This allows her to express her sexuality in a very uninhibited way. Better yet, he can reach around and stimulate her clitoris, so that she may well reach orgasm as they make love.

The two photos above are a very good illustration of how the divisions between sex positions can be quite artificial - in reality different sex positions merge into each other so it can be hard to distinguish between sitting positions, rear entry positions, and woman on top positions.

The photo below shows how the female partner may tighten her vagina - a tightness ensured by the fact that she has her legs closely aligned together while he enters her from behind and on top. This may not be comfortable for all couples, especially if he has a big penis, but for the less well-endowed man and/or the woman with a looser vagina, it can be a very satisfying way of making love.

The next two pictures hint at the man enjoying a really powerful and perhaps somewhat dominant sense of his masculinity as he enjoys intercourse.

And the pictures below show yet another aspect of the rear entry sex position. That is to say, the more romantic, loving and sensitive side: the couple kiss, feel the pleasure of close bodily contact between each other, invite each other to explore their bodies, and enjoy a more relaxed form of sex.