I forgot my password on an old iPhone and I don't want to reset it because I want to see what's on it. What do I do?

You will have to restore it using a computer. The process will erase the phone. If you have no back-up to your computer or an external drive, you will lose the data. Instructions here...

If you forgot the passcode on your iPhone, or your iPhone is disabled - Apple Support

You have no choice.

If anyone could see what was on an iPhone without knowing its passcode, the idea of using or having a passcode would be useless.

You can see what was on the iPhone by following the article linked above, then restoring its backup. Without a backup, its contents are gone.

If you mean the screenlock passcode, there is no way of resetting that without wiping the phone.

Sorry.