Cheers Jon, thanks for responding.
Sure you can ask: it's because the thumb is the only way to play a fair variety of chords and chord variations. For example, Fmaj7th (same as a standard barred F but with the first string open). Exactly the same shape but a fret up provides, F#7. Up another fret for a nice G6th. And so on up the neck. There are many other possibilities with different basic shapes which I'm only beginning to explore.
As you say, there are usually alternatives which can be employed at a pinch for a roughly similar sound. Looking at the first example, one can simply bar the whole thing for a plain F major, or leave out the bottom F, or play the first and second strings both open which gives a major 7th with a flat fifth (which is weird enough to not really have a name, but can sound good used sensibly - I use it quite often), and probably a few other things I haven't thought of.
I've been using workarounds like these for decades, because they are easier, and perhaps because I never really thought about using my thumb until I watched some really good guitarists in action recently and realised how important the left thumb is to them. But these alternatives are indeed workarounds and being forced to use them restricts my ability to play the notes in my head. So I'm working on it. (Not that I'll stop using the workarounds. They are good too.)
(Working on it 40 years too late, perhaps, but better late than never!)
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