Is it meaningful to raise frame rate, if so what is the best option?
Is it meaningful to raise the frame rate?

Usually, no. Exceptions are if you need to artificially generate slow motion using a robust tool like Twixtor. 

What is the mechanism of -r 60 in the above command? Since there are only 30 frames per second in the original video, where do those additional 30 frames come from?

They are simply duplicates of existing frames. Without the use of specific filters, ffmpeg can only duplicate or drop frames when the framerate is changed.

Is there any better option other than the above command to 'smoothly' raise the fps?

ffmpeg now has a motion interpolation filter, which will generate new frames. It can also just do a regular blend if that mode is chosen.

ffmpeg -i in.mp4 -vf minterpolate=fps=60 out.mp4