In general contact lenses are broken down into high water content and low water content lenses.
High water content lenses are lenses with more than 50% water, whereas low water content lenses have less than 50% water. Contact lenses with a lower water content dehydrate less on the eye than do lenses with a high water content, so they could be better for people with dry eyes.
Contacts with a high water content have a higher oxygen permeability. Therefor they are more suitable for longer wear. However high water content doesn't always translate into a higher oxygen permeability (the Dk value) with hydrogels, except if they are silicone hydrogels. Silicone is responsible for the oxygen permeability, and silicone is water repelling, (also known as hydrophobic).
As a result silicone hydrogels might have a lower percentage of water but can have many times the Dk of traditional hydrogel materials.
High water content lenses come with additional challenges for the manufacturer (and patient): They are less stable and tend to leave more deposits in the lens.
To summarize: If you have dry eyes, soft contact lenses with high water content can cause problems for you, since these absorb the natural supply of water (film of tears) of the eye like a "sponge". Silicone hydrogels are combining the advantages of lower water content lenses and high oxygen permeability.