If you don't need the new features of the latest macOS, and aren't troubled by any bugs in your current version that are fixed in the new version, you have no need to upgrade.
However you should be aware that Apple generally support only the latest 2 - 3 versions of macOS (formerly OS X) with security updates, so in 1 - 2 years' time you may find your current OS is no longer updated to fix any security issues that may emerge. So you might want to consider upgrading before this happens.
The macOS App Store will only offer you an upgrade to the latest OS version, but you can download and keep the upgrade installer without installing the upgrade - just quit out of the installer rather than proceeding, and it will sit in your Applications folder until you choose to run it. By doing this you can keep the option open to upgrade to Sierra after the next version is released, should you decide that you'd rather not go to the latest one for some reason. Alternatively I believe an Apple Store can install a non-current version for you if you take your Mac in to them.
If you have a full bootable backup of your Mac on an external disk, using a program like SuperDuper or Carbon Copy Cloner, it's straightforward to restore your system to exactly how it was before the upgrade, so you can try the new OS out and revert if you hit a problem. Alternatively you can install the new version on the external disk and boot from that, although bear in mind when evaluating the new OS that running from an external drive will usually be slower than from the built-in storage - also make sure you have a separate backup, just in case newer versions of apps like iTunes or Photos modify data on your built-in drive such that the older version can no longer read it.
I tried to find a definitive statement on the 2 - 3 version support cycle but it doesn't seem to be a published Apple policy, just the typical trend over the last few versions.