Many zeta functions and L-functions which are generalizations of the Riemann zeta function play very important roles in the modern mathematics (Kummer criterion, class number formula, Weil conjecture, BSD conjecture, Langlands program, Riemann hypothesis,...).

Maybe, Euler is the first person who consider the zeta function $\zeta(s)$ ($1\leq s$). Why did Euler study such a function? What was the aim of Euler?

Further, though we know their importance well, we should think that the Riemann zeta functions and its generalizations happen to play key roles in the modern number theory?