Preview thumbnails improve the user experience by allowing the user to scan the video for specific content. They also have a business impact determined by the bandwidth optimisation connected to users that can decide and quickly jump to the content they are interested in.
Appearing when hovering over the seek bar, preview thumbnails show a quick snapshot of the video content in a tooltip.
The most straightforward advantage of thumbnails is that they allow your viewers to scan the video for content, in case they are interested in a specific portion instead of the whole video, and then seek back or forward to the desired point.
Thumbnails have also a second advantage: it might benefit your bandwidth usage. As your viewers will more easily find the content they are looking for by seeking through your video, less content needs to be buffered. Although the advantage is limited to a bandwidth optimisation of only a few percentages, with growing consumption every percentage saved on bandwidth results in significant cost savings.