It's beneficial in that it effectively eliminates eye persistence motion blur. Think of a side scrolling 2D game like Sonic The Hedgehog. If you're running through a level quickly and there are say... stop signs that you're flying past, without Clear Action, you won't be able to read those signs. They will look blurry. This can't be fixed even with the fastest displays in terms of pixel response like OLED. OLED TVs will blur that sign, though not as badly as an LCD display.
With Clear Action, that Stop sign will look as easily legible and sharp as if you were standing still and reading it. I'll give you a few examples, done by Rtings themselves to give you an idea.
Sony 810C LCD motion blur
LG C6 OLED motion blur
Vizio P65 Clear Action motion blur
Disregard the slight after image there. That is a rare occurrence due to the P65's slower dark color pixel response, but in normal use is very hard to see.
Which of the 3 images look easiest to read? As you can see, Clear ACtion makes things in movement sharp. In actual, real world testing, it will look dsharper than even that image.
There is something people disregard called motion resolution. That is, in effect, the resolution of the image in motion. Without any picture effects, a normal sample and hold display like OLED and LCD's motion resolution is not EVEN enough to capture standard definition in motion. Hence why everything is a blur. With flicker techniques like Clear Action, images can have more than HD to more than Full HD levels of motion resolution. That means, in motion, the entire image looks nearly as good as if nothing was in motion. You could still see ther smaller, finer details in all objects.
That to me, only enhances the 'feel' of high definition, as there is no sub-HD image present. People fight for high resolution, yet they easily decide it's ok to give up any and every detail in motion. That to me goes against what makes HD so enticing.
As you may know already, there are tradeoffs with flicker/strobing techs like Clear Action. The image brightness is cut down basically in half, so if you're used to a high level of brightness, the initial hit may be a bit much. In SDR it is easy to offset the brightness loss by raising the backlight level. So say, you're used to backlight 50 in SDR without Clear Action, you'll want to change the backlight to 100 when Clear Action is turned on to mitigate the light output loss. You do not wanna do this in HDR however, as that messes with the medium tones.
The other tradeoff is the initially distracting flickering of the display. This is and will be the thing you need to let your eyes adjust to, as at first it will most than likely bother you. That being said, just as your eyes adapt to a dimly lit room, where you couldn't see anything in the dark, and awhile later you could see well, you always wanna let your eyes adapt to the flicker. Trust me when I tell you, it looks gross at first, but the longer you have it on and use the TV with Clear Action on, the less and less you notice the flicker to an eventual point where you don't notice it at all. But, you will ALWAYS notice the benefits of motion clarity.
As for Clear Action and 120hz content, the benefit there is very minimal, as the display is already flickering at the same rate as the display is refreshing. At best it will very slightly clean up the image, but honestly it's mostly just a slight brightness reduction at 120hz, and don't need it to be on. Its main benefit is for 60hz signals. 120hz is minimal, and 24hz signals, it won't work at all as if the TV disengages it even if you turn it on.
edit: Ah yes, there are various types of motion blur.
Pixel response blur
Eye tracking/persistence blur
embedded motion blur
If something like certain video games have embedded motion blur, Clear Action will not eliminate that. There are plenty of video games that enable motion blur, which only compounds the blur problems on these displays without Clear Action. A good example is Wipeout Omega Collection which has a motion blur option. You can see that this will exists even if CA is On.
That being said, video game motion blur is particularly beneficial to 30fps games, as it smooths out their perceived movement. Ratchet and Clank on PS4 is a fine example of incredibly used embedded Motion Blur.
Pixel response blur is shown as a trail after image in motion, as what you see in the Vizio test image above. The slower a display in pixel response, the longer the trail. If you've ever seen a Sony PSP, you'd know about the horrible pixel response in its blacks, that smear across the screen in motion.
Ah yes, you shouldn't use Clear ACtion for movies. It will mess up the 24hz cadence 3:2 pulldown where it applies, and movies/TV tends to have its own form of inherent blur caused by either the cameras used or in post for CGI, etc.