Suppose the wheel is true, all spokes have tension above some minimum, but some spokes have higher tension than others.
Why is the uneven tension bad? Other than offending our OCD perhaps.
Spokes work as a system, is essence a team. Imagine trying to build a football team with a line having a couple of good players, and some not pulling their weight.
Also, modern rear wheels have quite a bit of dish, so half the spokes are near the maximum acceptable tension, and the other half near the minimum allowable. So the working band of tension is fairly narrow, yet uneven tension means that the band will be wider, meaning some spokes will not be within an acceptable tension range.
So IMO, it's not about even tension per se, but the benefits of even tension, and the drawbacks of uneven tension.