This type of masturbator provides a hard encasing around a soft, textured sleeve. This allows you more control over your sensations; some of these masturbators allow you to squeeze them, creating more pressure and a tighter feel on the penis.
These masturbators are also great for longevity: the soft material located inside of these masturbators can disintegrate and break down over time from use. The hard external case provides a level of protection for this material that soft masturbation sleeves just don’t have.
Some of these masturbators are also incredibly discrete! Their aesthetic means that it would be hard for anyone to tell what they are.