Originally Posted by
BloomingCyclist
If the tension is not high enough on the spokes, the tension gets too low when the spoke is at the bottom as the wheel rolls (when the spoke is compressed as it bears the weight / force). and they can loosen.
If you omit the comments in parenthesis, you've got it right.
Although technically a bicycle's spoke network holds a wheel in suspension, it would be more accurate to describe a wheel as a structure that hangs from the spokes instead of standing on them. As you note, as a wheel rolls the spokes achieve their lowest tension as they move through the 6 o'clock position and distribute the loads around the rest of the network. Having a wheel with low tension in one or all spokes will create all kinds of problems, including noise and truncated spoke life.