They are a simple and relatively cheap way to slow down drivers after neighborhood complaints, schools opening, or near parks. Officers I have talked with say their departments are stretched too thin to spend officers time on speed enforcement and from what I see it seems so, I see far fewer officers using radar than just five years ago.I'm suggesting those portable speed signs are likely capturing more data than vehicle speed. I have no proof, but to @actaeon277's point, it seems unlikely a department would pay for multiple units, maintain them, shuffle them all over town if their only purpose was to display a driver's speed. They clearly save data because departments sometimes declare 'high speeds', 'avg speed', etc. that are captured. Maybe they are only used for altruistic purposes. Color me skeptical.
I suspect they do keep track of speeds but I do not see any cameras on them for additional data.