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Why Using Labeled-Unicast traffic ( single MPLS label )
BGP-LU is used where you need to join multiple networks together (e.g.: running distinct IGPs) while still being able to provide a transport label between any two nodes.
A couple of use cases that come to mind:
Large cellular backhaul network - may have 10s of thousands of base stations. It wouldn't be feasible to have these all participate in a single (say) OSPF area and distribute link-nets and loopbacks. Using BGP-LU you could break it up into regional IGPs, but still establish end-to-end LSPs across regions to a centralised head-end
Merging two existing networks together - you're a large ISP running OSPF as your IGP, and you acquire another large ISP running IS-IS as it's IGP. With BGP-LU you can create LSPs across these two networks without having to perform some unholy route re-distribution between two link-state protocols.