From what i have heard you can use a jungle swarm to protect your most presious units.
This is the way it works: Lets say you have your slann and a big block of temple guards. This is often enough your armies pride and joy, the lot have stubborn, immune to psyhology and ledership 9. The fact is still that if you would say to loose a combat and recive a bad roll the whole lot will flee and the enemy will most likely be gunning after you. Losing that roll would be disasterous!
By putting a jungleswarm behind your big block as a "speed bump" you ensure your company to stay safe from anyone willing to hunt them down.
I can't say this is a must, it's just a way of avoiding disaster if the unthinkable would happend