This question can certainly be scientifically approached. Let's attempt to improve and reopen it...
Reads like it's asking for understanding personal habits of other people. A more sociological and psychological question about how the mind works and why some numbers make people drool over another.
- What's the importance behind boot time metrics?
- Why do we care how fast it kicks up?
- Why do reviewers hark on about it?
- Why do other people turn off their computers when I put mine to sleep or not at all?
Which leads to spitballing:
- It's the first metric you experience on a new computer.
- People or companies turn off their computers when they're not in use for long periods.
- New users can understand this easily in demonstrations.
- Once it was running clock speed, but now boot time is the latest thing for manufacturers to unzip and flop out to sit on the card table looking you back in the eye.
It's the same as asking why car reviews always emphasise the 0-100 acceleration when drivers rarely have legal opportunity to test this out, don't care for it, don't use it to such speeds and other matters of personal consultation.
So why do reviews now seem to be focussing on this aspect when they didn't before? What changed really in the user and corporate environment where people have always been hibernating, sleeping or shutting down to their own preferences or limitations? Why now do we care about how fast it boots up?
And now we're onto marketing.