Why would anyone want class AB amp when class A always sounds better ?

New Class Ds (e.g.NAD etc)..it's the future, like it or not. Have a listen, go on.I've had a range of A, A/B and early Ds over the last 50 years. Not going back away from D now - sound, size, heat, weight, WAF - all ticked.

The only thing that matters to me is the interaction with the speaker I'm connecting it to. Over the years I always seem to gravitate back to class A. It's a finer more complete presentation to my ears. The whole notion of everything on audio being black and white across the board is what makes the used market ripe with choices.   

Class A amplifiers don’t always sound better. It depends on the headphones or speakers you’re using. Headphones and speakers with superior transient response (able to keep up with and resolve complex music) tend to sound better with Class A. But not all are made equal. This means you could have a Class A amp that performs worse than a Class AB - although, if designed well and without compromises, Class A is the gold standard.

This is a great resource:
https://circuitdigest.com/tutorial/classes-of-power-amplifier-explained

I recently added a Gustard X26 Pro to my Benchmark HPA4 | AHB2 mono. It replaced the Benchmark DAC3B (still keeping it). The sound of my system was transformed to that of the Gustard. The HPA4 and AHB2 do not seem to add any flavor.

It’s the ultimate chameleon. What the source puts in is exactly what the AHB2 dishes out. 

You're not going to operate Class A on solar panels and it is the opposite of going green. What it can be when properly designed is a fabulous sound that's very involving, Hey folks all Class A aren't created equal, just like Class D or Class AB for that matter.