What exactly is purpose of flachette for shotgun and when to use

It does not do extra damage vs infantry. Your shots go through infantry, if 2 enemy are right behind each other 15m away and you use buckshot, first enemy dead, 2nd full hp. If you use flechette,  first enemy dead 2nd enemy -50 hp. It does this for all enemy soldiers within its range. So only use when you are fighting indoors, as this is where most likely enemy's will be stacked or lined up.