Question

why are things grouped together

Answer 1

things grouped together because in some type of group we can easily get like-( articleof any property or thing etc) other wise it take time to find that product

Answer 2

To make the study of these things easy, these things have been divided into few groups in such a way that things in the same group have similar properties.  

For example, in a shop, soaps are kept

together at one place while biscuits are kept together elsewhere.

Even among soaps, bathing soaps are stacked separately from

washing soaps.