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Abstract: The short answer to the title question is “You do!”, if you are interested in understanding major observed features of astrophysical plasmas and energetic particles. The significant impact of turbulence in the field of neutral fluids is very well accepted, but less so in space and astrophysical contexts. Here we review some observed properties of plasmas that apparently depend on specific effects of turbulence. The focus will be on the solar wind plasma, widely regarded as an accessible natural laboratory for study of turbulence and other basic plasma physics phenomena. A traditional view is that the solar wind emanates from the corona carrying a broad spectrum of small amplitude outward propagating waves. More detailed observations have revealed systematic effects that are inconsistent with a wave picture, but yet can be accounted for by a turbulence cascade. These feature include excess magnetic field energy, a progressive mixing of inward- and outward-fluctuations and substantial proton heating that is inconsistent with adiabatic expansion. The cascade rate of the turbulence has also been directly measured and has been linked to the formation of coherent current structures. A number of these results have emerged in the past two decades and have substantially changed the way we look at the state and dynamics of space plasmas. At least for the solar wind, the accumulated evidence supports a characterization, not as noninteracting waves, but rather as a dynamically active turbulent medium. Brief mention will be made of connections to the plasma physics of dissipation, and to the implications for energetic particle propagation.