Crossovers are often a low dribble, because the ball needs to be kept tight and in control to keep it from being stolen. That applies to standard crossovers, through the legs, and behind the back.
Low dribbles require a harder dribble. That is because the ball does not fall from as high up, so not as much gravity acts on it. The low dribble is also more of a horizontal dribble than a vertical one. So in order to keep the dribble alive and the ball bouncing, more force is required.
The best way to get better at dribbling the ball low is by practicing as much as you can. The more you practice, the more comfortable you will become with it and the more consistent you'll be as well. See how long you can dribble for while staying low, or how low you can comfortably keep the dribble alive at.
Low dribbles are harder for a defender to steal. The ball is in a vulnerable position (out in front) for less time as well as in a spot that is not easy to reach. Low dribbling is also effective when a player is driving into the paint. That is because once an offensive player gets there, it is very congested and maintaining control of the ball is important. The defenders in the paint are also usually taller positions, so it would be harder for them to reach down to steal the ball.
A low dribble is also often used for learning how to dribble better. Young players do drills where they need to dribble the ball low to the ground to help them with controlling the basketball. Sometimes the dribbling training will also involve weaving through cones or obstacles. That way those in training can practice a low crossover.