Er, you don't need to. Plus you sort of can.
Let's say that I have a unit of zerkers 18" away (or further if you have double lash - but I guess then you don't have a juggernaught) from an enemy unit at the start of the turn. They move 6" towards the enemy and I lash the enemy unit to just within charge range (while ensuring that any nasty surprises like hidden power fists are well out of the way). I then charge my zerkers in, cut the enemy unit into little bits, and massacre forwards another D6". Maybe I stay locked in combat or get to consolidate into something, so my unit can't be shot.
So my unit has:
- Moved faster than it could have if it had jump packs or a transport
- Killed an enemy unit
- Maybe is hidden from return fire
The great thing is that, usually for assaulters, you get to pick where the close combat actually takes place. You can put the guys you are going to charge somewhere that it would be convenient to fight them. out of cover, near another of their units so you can get a consolidate, whatever.