For determining the centre of mass of a body,why is a body considered as being composed of multiple small mass elements?

(N/A) rigid body consists of a large number of particles (atoms or molecules),and the distance between these particles is extremely small. Because the body is continuous,it is mathematically impossible to sum the product of mass and position for every individual atom. Therefore,the body is treated as being composed of $n$ small mass elements $(dm)$,allowing us to use integration to determine the centre of mass: $\vec{R}_{cm} = \frac{1}{M} \int \vec{r} dm$.