Thumbnails in my opinion are extremely important not only for gathering attention but just because have they have become a standard, a video without a thumbnail put next to a video with a nice thumbnail will loose out in most cases simple because not having one gives off an image off poor quality. Thumbnails can be important for conveying the message or topic of a video in a single image and can encourage people to come watch just based on that image alone
Thumbnails for me are really important there the first thing the viewer see its kinda like Tinder if you have a bad photo of yourself more chances are they will swap left rather then right it's all about engaging with your audience and making them want to click on the video!
I think thumbnails are really important. Its the first thing someone will look at in order to decide whether they'll click your video or not. So its probably a good idea to focus some time into making a decent thumbnail for your videos
A thumbnail is like a billboard. There are some billboards that have been known to cause accidents because they were so distracting or well done. If you can have the best SEO and video, but if no one is clicking on your thumbnail then you're at big disadvantage.
I used to think that thumbnails weren't important and was growing fine without them, but there is only so far promoting will take you. The main key to success on Youtube is search and suggested videos and to get the clicks you need, you really need a good thumbnail. Bit unfortunate really as mine are terrible for the most part still.