Complicated behaviour is common in solutions to the Navier-Stokes equations when the viscosity is small. The mechanism of the complexities is obscure even today. Here is a reason why numerical computations play an important role in making a qualitative picture of the Navier-Stokes flows. The purpose of this paper is to explain, through examples, necessity of numerical computations in the analysis of the Navier-Stokes equations. More specifically, we consider two dimensional freely decaying flow and the Kolmogorov problem.