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I guess you can also say that well taps are placed in order to ensure the substrate/well is kept at vdd/gnd potential or very close to it. Switching circuits dump current into well/substrate and if there is a high resistance between substra/well and the vdd/gnd grids the substrate can be at different potential than vdd/gnd (this cause potential latchup and all sorts of other effects). More taps reduces the resistance.