You can use KML or KMZ data like any other format, and access its rich styling capabilities. The format is heavily geared towards client-side viewers, however, and some of its capabilities are not compatible with OGC web services. For example:
Balloons: some KML objects come with a balloon. If a user clicks those KML objects, a styled balloon appears. If you serve KML data through OGC web services, those balloons are not available.
KML screen overlays: screen overlays are often used to display a legend in the top left of a client view. Screen overlay elements are inherently incompatible with OGC services such as WMTS: the screen overlay may appear on every tile if you serve KML data with a screen overlay from a WMTS service.
Self-updating data: OGC services do not support the polling of network links to retrieve data updates.