Given a pair of a compact Lie group and its quaternionic representation, physicists consider a gauge theory in dimension 3, and associate its Coulomb branch, which is a noncompact hyperKaehler manifold possibly with singularities. Recently I propose its mathematical definition, partly with Braverman and Finkelberg, as a holomorphic symplectic variety. This definition gives a new class of symplectic varieties, which seems interesting in its own mathematically. In this talk, I will explain my motivation of the definition, based on a hypothetical topological quantum field theory associated with the gauge theory.