Hello Samrar,
multiple spanning tree, or MST, is mainly used to reduce the overhead that spanning tree creates when running a large number of VLANs. Let´s say you have ten VLANs, without MST, each VLAN would have its own instance of spanning tree, which can cause a.o. an increase in CPU overhead. The idea behind MST is that in most topologies, you would have e.g. the same root switch for all your VLANs, so there is no need to run a separate instance of spanning tree for each VLAN. MST ´bundles´ spanning tree instances, so in the case of the ten VLANs mentioned before, you would effectively have only one instance of spanning tree for all ten VLANs.
Have a look at the document below, it describes various scenarios where MST is typically used:
Understanding Multiple Spanning Tree Protocol (802.1s)
http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfc.shtml
HTH,
GP
