On the 737NG, FLCH is fine to use on approach. You can use it to descend to the platform altitude you capture the G/S from!
You usually find you can't get adequate descent while slowing down while vectoring towards an approach in VNAV PATH, so LVL CH is ideal to simplify things. It is indeed a pitch mode, the logic is that when you select it you get an aircraft that either descends on closed throttles or goes up in N1 climb.
It does not take much mental idea to work out what is going on. Simple types would have an IAS button and you move the levers to go up or down.
It works just the same.
In this mode, you have the speed window open so altering the speed alters pitch. slowing a 737NG down near up speed and it really doesn't go down much without the "Stick of Shame".
No idea about the 777, but common Boeing philosophy should apply to a mode so easy to use like this.
Why in god's sake these pilots were using this mode to descend on a final approach though is a different matter. Then again I can't grasp why airlines don't let pilots use the damn thrust levers either.