What is the significance of Sphingolipids in human?
There are multiple known functions of sphingolipids and their metabolites in humans, although the roll played in the structure of myelin might be the most important and well understood.  

However, according to this Wikipedia article, sphingolipids also play a roll in cell recognition and signalling, among other specialized functions. Additionally, some of it's metabolites have been shown to perform numerous functions. From the above article:  


  Recently, simple sphingolipid metabolites, such as ceramide and sphingosine-1-phosphate, have been shown to be important mediators in the signaling cascades involved in apoptosis, proliferation, stress responses, necrosis, inflammation, autophagy, senescence, and differentiation.  


The bottom line answer to your question is that there are many other functions of sphingolipids in humans, particularly if you include it's metabolites.  The article cited above is well referenced and should serve as a good starting point to investigate (or just enumerate) the other functions of sphingolipids in humans.