The importance of keeping up with latest version
An important reason to upgrade is to continue to obtain the latest support for SQL Server. Once the mainstream support for SQL Server ends, there will be no more service packs or cumulative updates bringing fixes and updates to your current version of SQL Server.
As you can see from the following table, just three SQL Server versions are supported as of now (summer 2020) and those are SQL Server 2016, SQL Server 2017, and the current SQL Server 2019:
Although many of the older versions are not supported anymore, this does not mean they are not used in production environments. The preceding table does not list all the SQL Server versions. There are even older systems that can be used today, but those fell out of support a long time ago. Also, note that SQL Server 2008 and SQL Server 2008 R2 share an end date for mainstream support, although...