This would be fine in the main forum or here, really, since it's more of a "how does this even work?" question rather than a "how do I best make use of this?". But either way we're not too persnickety about the line between the main forum and the strategy forum. Anyway.
The info the scout makes available is complete as of the time the scout is there. Leaving a scout "picket" lets you maintain updated information on the planet, which is often very helpful.
Particularly, if you have lots of threat ships flying around (as in a Cross Planet Attack, for instance), having pickets on all planets adjacent to yours will let you use the Detected Threat galaxy map display mode to see where the AI is in position to hit you. Though if the threat is a couple planets back you won't see it unless you've got pickets out that far or it advances to a planet that does, etc.
