There can be several reasons to upgrade vSphere to the latest version:

• Extend the support and the life cycle of the product: VMware vSphere 5.5 will have an extended support and will reach end of general support in September 2018, VMware vSphere 6.0 on March 2020, and vSphere 6.5 on November 15, 2021.
• Have a new product: It provides new features but also new hardware (and other new  software) may require or have some benefit from this version.
• New infrastructure functions: Such as the new high availability features; we will discuss all those functions in upcoming chapters.
• New security functions: Some are really cool and unique, but data at rest protection using encryption could also be possible not only at hypervisor level (in this case only with 6.5) but also at storage level (also vSAN now has this capability).
• Storage benefits: If you are using vVols, you can now have a native replication support (of course, if your storage vendor supports it in vSphere 6.5). If you are using vSAN, the only way to upgrade it and have new features is to upgrade vSphere.
• New web client (vSphere Client): Finally, we have an HTML5 client (not 100% complete, but very close, at least for operational tasks) for the vCenter Server graphical management. For the ESXi host, there was already (starting from 6.0 Update 2). Note that you can add both the HTML5 clients to the previous version using Flings software (https://labs.vmware.com/flings).
• New vCSA: The new virtual appliance for vCenter is definitely the first choice, due to the full capabilities and also for the new functions.