Attention is a kind of surplus mental capacity that we have, which isn't specialized, but can instead be directed to tasks as needed. Ironically, we also use the term for the dedicated mental system which directs this extra capacity, which makes talking about it a bit more complicated.
Most of the stuff we do, our brains just kinda handle for us. Walking is usually like that; it's an incredibly complex feat of dynamic balance, movement planning, and adaptation to changes in the environment, but it rarely takes any conscious effort on our part. Conscious effort is directed attention.
I mean TVs have volume buttons but also a mute. It's nice to be able to use volume to set a specific level but then also quickly toggle between that perfect level and silent.
It's not something you strictly need a physical button for, but the way they implemented it on old iPhones was nice. It was a physical switch rather than a button, and it looked different in the two positions--the slider under the switch was red on one side and black on the other. (or maybe silver, i forget, but it didn't stand out the way the red did.) So you could tell at a glance if it was muted as well without turning on the screen.
The new button seems like a step back from that to me, but if you don't use the silencing feature then a reprogrammable button is maybe more useful to you.