One of the advantage that the Samsung Galaxy S8 has compared to its predecessor is that it brings an Electronic Image Stabilization (EIS) in 4K video recording. In the Galaxy S7 edge there was an EIS option, but it wasn't available in UHD/4K, only in resolution below it. Regardless of the rear camera having an OIS, you could clearly see that the footage was still bumpy. OIS is good for compensating for small hand movement, but the shakiness in walking while filming is just to great for it and I believe this is why an EIS does a better job in this aspect. EIS correct the movement by digitally moving the image on the opposite side. Of course it relies on the amount of extra pixel space that you record the images on. The smaller the area of recording compared to the actually one that will output, the more room the EIS has to correct this camera shake issues.
I think that the main reason for this is the due to the new rear-facing camera module, it's either the Sony IMX333 or the Samsung ISCOCELL S5K2L2.
Many users prefer shooting in 4K even if they don't have a 4K display at home. The reason for that is that you want to maintain your precious memories in the highest quality possible, because in the future, if you had a 4K display, you can enjoy looking back at those memories in very high details.Of course 4K videos weigh much more than full HD videos in terms of file size, so some users prefer not to shoot at this resolution.
Nonetheless, what's important is that if you decide to shoot a 4K video, you can rest assured that the EIS will do its job just fine. Here are two video shot by gsmarena.com that demonstrates the effectiveness of the EIS in 4K 2160p video recording. You can see that the Samsung Galaxy S7 edge is very shaky, unless the photographer is not moving. The Galaxy S8 video is much smoother compared to the S7 edge and you can see why you want to have this EIS feature turned on when recording 4K videos.
Here is the Samsung Galaxy edge 4K video sample:
Here is the Samsung Galaxy S8 4K video sample:
Can you notice the difference?