EAS is a function of dynamic pressure (q), drag is also a function of q (=.5 x density x TAS^2); (almost) all structural loads are first of all a function of EAS.
TAS is what you need for navigation so is generally quoted in the performance data as a function of altitude and power (and in bigger aeroplanes, weight).
IAS is the only thing indicated in the cockpit, so used for any piloting task as the reference speed, whether that's climb profile, stall, approach, flap limiting speed...
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