Number is a simple abstraction: an exercise in conceptualizing a particular part of human experience, - the amounting of stuff and the relations of various amounts.
Its utility shines the most in the practice of measurement: determining, manipulating and comparing the amounts of stuff.
Numbers also useful as a stepping stone in a learning journey, allowing an individual mathematician to transition to using other, more powerful abstractions (like variables, polynomials, sets, functions, vectors, fields, etc.).