Plastic push-fit vs.compression joints

I want to know the use scenario of both, I have a 40 mm drain pipe in the ground and 40 mm shower trap pipe so I want to connect them both. I guess for my use scenario it might not matter much due to low pressure, however that raised the question if I should nearly always choose compression instead of push-fits?

Also why do compression joints sometimes come with the additional hard/flat washer along side the coned rubber one, sometimes they don't?