Yes, as a matter of fact they do.
In order to watch any video whether it is on Youtube, DVD or Blu-Ray, the video itself must be decoded. That takes processing power either by the graphics core or by the CPU. Video encoding and decoding tech has advanced since you bought your laptop. Currently used codecs provides very good quality and better compression (smaller size files), but they also take more processing power to decode. Or a graphic card with the appropriate built-in hardware to help decode the video without much trouble.