If you are running a mid-2007 iMac, you have probably only been able to upgrade up to OS X El Capitan. This can cause problems such as:
- No longer receiving security updates
- No longer being able to download hardly anything from the app store
- Not being able to use essential apps like Pages, Numbers, or Keynote
As a developer, I've also run into a couple of other problems:
- El Capitan only supports Xcode versions up to 8.2 - this keeps you from using most pods and some newer Swift syntax.
- Google's Flutter platform will not run (as of 1/3/19)
Clearly, it's a good idea to upgrade to Sierra. The problem is, it isn't supported for your Mac's hardware.