Shields take an action to "don", i.e. start to receive the benefit of the shield (2 AC for basic shields, other effects for magical shields).
Can you pick up a shield and hold it without "donning" it? If you could, I would expect you would not have to use an action to pick it up (free object interaction) but would also not get any benefit from the shield until you did use an action to "don" it.
The use case is that JC says shields are a valid target for disarming. In that case, the shield would drop to the ground, and the answer to this question would determine what the shield's owner could then do (if, for example, they did not want anyone else to get their shield but didn't have the time to use their Action on donning it again).