There are a few reasons why you might want to download and install old versions of Mac OS X or macOS. Perhaps you are relying on software you have discovered doesn't work properly in the newest version.
If you purchased your iPhone within the past few years, it's likely to be unlocked, and free to use with any SIM. If your iPhone is older than that, it might be locked to a particular network/provider. But that doesn't mean you're stuck with that provider forever.