Spine-leaf design was created to address data center issues. The previous designs, 2-tier and 3-tier were used before Spine-leaf came out. The challenge with the two previous designs is spanning tree issues and first hop redundancy configuration issues. With spine-leaf those issues are removed. The reason is because of the new technologies in the spine-leaf design which eliminates spanning-tree protocol.
Spine leaf consists of only the spine which is the layer 3 section, and the leaf which is the layer 2 section. It’s a full mesh design so that every leaf connects to all spines. It decreases latency with less hops and takes full advantage of all the bandwidth. It is also beneficial for networks with more East/West traffic than North/South traffic. Data Centers have a lot of East/West traffic. See below for an example:
Ken Hagen CCIE #56460 