Scouting usually precedes Plundering, so if you find from your Event interface that you were just Scouted you better start Garrisoning those Heroes and troops in your City Defense Office.
Scouting is a means by which you can get a sneak peek at the defenses surrounding an enemy City before you Dispatch a Hero or two to Plunder it; it's no use attacking a City whose Defenses will undoubtedly decimate yours and equally foolish not to attack a City that's not defended at all. Scouting often does not provide a complete picture of the units stationed in another City's Defense Office; the accuracy with which you might gather this information depends on the level of the Hero whom you send to Scout.
Scouting a City requires a Hero to physically walk all the way there and back (after which a separate trip must be made to finally Plunder), which takes time as well as Food, a burden than can become quite heavy if the targets of your espionage are located in another Civilization (as they should be!). The Mall Treasure Clairvoyance, released in the Dante's Circle Expansion Pack, allows for instant information on an enemy City with no need for physical marching.