How does a clock have a constant speed, but not a constant velocity

Speed is just half of the story.

Explanation:

Velocity is a vector measure. As a vector, it has a magnitude (absolute value), called speed, and it has a direction. The direction in one dimension is just the information about the vector pointing "to the left" or "to the right."  In two dimensions, the vector has an angle (in addition to speed) in the range of 0 to 2pi. In three dimensions, it has two angles, in addition to magnitude. etc.

So, by saying the speed is constant, we are only told the length (aka magnitude) of the vector is not changing. But the direction can be changing happily! Think of a satellite orbiting in the Earth - it has constant speed, yet its direction keeps changing to follow a round trajectory.  Same applies to a clock with hands.

this allows people to make the best decision or pursue the best option. when alternatives are weighed using their advantages and disadvantages, the option whose disadvantages outweigh the advantages (based on a qualitative or quantitative analysis) are shunned while those whose advantages outweigh their disadvantages are pursued.