for example when you have a campus setup.. two datacentres close to each other with enough bandwith in between.
in this case you need a stretched vlan to vmotion virtual machines from the one datacentre to the other..
another example when your esx hosts are not close to eachother in a datacentre for example divided in several server rooms and you need to vmotion to eachother als oa stretched vlan is necessary.
but when your esx hosts are connected to the same physical swithes you can even use local adresses even without a gateway defined..
