Why do modern hockey pitches look wet?
To answer this question in a sentence, modern hockey pitches use a water-based artificial turf. There are two main reasons for using this new kind of astro turf...
The water based system allows for improved ball rolling and bounce, mainly to improve consistency. There is no sand infill.
The wet pitch means there is less friction between the synthetic fibres and whatever is passing over it, including people sliding. There is therefore less risk of scrapes and burns and also other injuries such as knee twists which occur from players avoiding falling and sliding which is no longer a problem.