In-betweening has been an very important thing in animation because helps make the movements look smooth and realistic, Most Western animation is mostly done on 2s and 1s separately.
but the framerates all rely on the in-betweens, timing and spacing.
Most movements (Normal, slow and fast) is animated on 2s
The very fast movements are on 1s.
Russian animation is also animated on 1s and 2s.
Anime however, is usually done on 3s.
for the anime movies is done on 3s 2s and 1s.
I think 2s and 1s are more fluider than 3s because 3s can sometimes have an jerky movement except for 1s and 2s
1s and 2s is much smoother than 3s.
Back in the silent era of animation, Pionners like Winsor Mcay did very great, smooth and realistic animation,
Like this video for example:
https://www.youtube.com/watch?v=K8qow7jTyoM (go to 8:17)
When the animation industry was involved back then, most of the animation was done without in-betweens.
Like this short of Colonel Heeza Liar for example:
https://www.youtube.com/watch?v=djyB2SEKqo0
When animators did in-betweening for the first time in the industry, they didn't do timing and spacing properly and it's unusually fast at some places without spacing it properly.
Like this video for example:
https://www.youtube.com/watch?v=oEamAzZwpJo&list=PL6mBZHANdP4dqK_aw1dEYk...
When animators actually improved timing and spacing with the in-betweens it actually looks good so far.
According to Timing for animation by Harold Whitaker, Fast movements suit animation best for a lot of reasons on particular movements and easing in and out suits best on animation very well and help to make it look realistic.
In-betweening is very important in animation and same thing with Timing and spacing.
