The paths laid with the path tool in the editor are used by the AI for automated pathfinding. To find a way from one location to another, the bot looks for a way using these laid out paths.
Not laying any paths to a map will result in the bot trying to take the shortest route, which may be through walls or other obstacles. The effect can of course be used to make the bots seem lost.
The paths should be laid out in the simplest way possible, because too many pathfinding points slows down the pathfinding.
Note: The nodes check for terrain objects only when the map is loaded so if you suddenly add a wall between two points the bots won't realise it until the map i reloaded, but normal objects are checked right away(ie. you could make a maze of pushable crates and the bots will navigate it fine regardless of how it changes, assuming there are enough nodes).