The leak off mod as already stated stops air being trapped at number one injector which can cause starting issues and running problems.
If the leak off pipes are cracked they will allow fuel to drain back to the tank overnight causing starting problems and again air being trapped without the mod.
It's normally worse if you park the van uphill. If you're doing the leak off mod, you will need to purchase a coil of leak off pipe anyhow to reach the front injector, so with the left over pipe, renew the small lengths between the injectors and Bobs your whatsit !