We have assumed the notation '1' for any thing in this universe which is single so that we can give its numerical value and can study about it. An to establish various theories to know about the wonders of the universe or science behind the natural phenomenas occuring. See, to study anything we have to make an assumption to study about it and analysed it after an assumption then only we can start studying about it.If Newton won't have assumed that each and every object exerts an entropic force towards all the bodies surrounding to it, then today we won't have any kind of Gravitational force or law like that.
I hope ur getting my point . I mean to say is that to have a study over anything we should assume its refrence point to begin its study.
Thats why '1' is called one which is alone united as single individual which is not two.We have assumed to have 1' as one to study mathematics , Physics etc.
I hope u got it !
Plz mark as best if u liked it !