Vsync synchronizes the frame rate of the game with the monitor refresh rate. That's it's most basic function.
Yes, as I understand it (and I'm not an expert) is that Vsync is enabled to prevent screen tearing which is where the monitor and game are out of sync. If the game runs at a steady 60fps then there are no problems. The trouble comes when and if it dips below that, in an attempt to keep the game and monitor in sync frames will be dropped periodically which results in a noticeable stuttering effect.
There are fps limiters such as this one which when used in combination with Vsync OFF seems to stop the stuttering by removing the fps cull. So even if the framerate dips considerably (as low as 21fps in parts in JK's Whiterun) the game remains smooth and stutter-free. The downside is obvious - by removing fps culling the game will go out of sync with the monitor and you will potentially get severe screen tearing in places.
So its a trade-off, loss of frames/stuttering vs screen tearing. It seems Bethesda went with the former as the least worst option. Now personally I have an adaptive sync monitor (Freesync) so tearing doesn't happen for me which in combination with the above fps limiter and Vsync off makes it the perfect solution for me, others will possibly experience different results.
Also, your 30FPS comment is a matter of opinion and is subjective to the individual user. I know when Skyrim drops to those levels I most definitely can notice the gameplay is not as smooth nor fluid as it is at 45FPS+. 45-60FPS, imo, is the sweet spot for Skyrim. This is, of course, subjective as well. ::
As so often with modding my experience is different, at least with the above mentioned and greater than 30fps. Below that its not severe but there a slight.. flickering or something. Possibly because the eye can separate individual frames below 25fps I believe, above that persistence of vision in the retina means that they're merged into a single moving image as I understand it.
Edited by soupdragon, 05 November 2015 - 06:04 PM.