why 23.976 fraction in composition setting ?

That is a standard frame rate for 24P video on NTSC systems. The fractional value is so broadcasters can conform to the 29.97 FPS frame rate for standard television broadcast. The fractional value was implemented when NTSC countries, countries with 60Hz power, started broadcasting color so that the color information could be passed through the transmitter to the TV sets. They needed to add two extra scan lines and include something called color burst so the frame rate needed to be slowed down to send the extra lines of information. When PAL countries, countries with 50Hz power grids, went to color tv they decided to add the color information by removing a couple of picture scan lines so they didn't have to change the 25fps frame rate. In both cases, a TV frame consisted of a pair of scan lines sent at twice the frame rate.

If your content is intended for Television you have to comply with 29.97 FPS to make your video broadcast-legal. If you want to have the look of 24fps (motion picture film standard) then you shoot and edit at 23.976 then add 3:2 Pulldown when you send the project for the final render. If your content is only intended for the internet then you can shoot, edit, and deliver at 24, 25, 30 or even 60fps. I'm not going to get into the requirements for high frame rate video now.

The point of all this is that if you are not an expert in video formats and know exactly what you are doing you had better stick with the standard presets and make sure that you pick one that is square pixels. Non-square pixel projects also require you to have a thorough understanding of video standards and are not intended for delivery to the public anymore. Personally, I think all non-square frame sizes in the composition presets should be placed in a separate category and clearly labeled as Camera original Only - Not intended for delivery or maybe even removed from the options. I haven't set up a NonSquare composition in the last 20 years, but I've answered hundreds of questions from users that don't understand video and get confused with their output is distorted.