It is assumed that everyone needs to do exactly 40 hours of work per week, but ask yourself this: for every person in your company, what do their next 40 hours look like? 40 hours worth of HR policies need to be created. 40 hours worth of sales calls need to be made. 40 hours worth of snacks need to be ordered for the office. 40 hours worth of website text updates need to be made. 40 hours worth of UIs need to be designed. 40 hours worth of code needs to be typed in. Isn't it strange that all these vastly different tasks take the same number of hours to complete in a week? My guess is that chattin' is what takes up whatever time remains; people are required to pretend that they do 40 hours of work every week, so they come up with their own way of filling the time. Planning is always valued (and a good idea!), so if you report "yeah I spent the week planning for our Q3 XXX" then it sounds like the money spent on your salary was worth it, and it continues to pay.
As a software engineer, I've never had the problem of not having enough work to do. Tasks are always added to the backlog at a faster rate than they are removed from the backlog. But I feel like a lot of other jobs aren't like this, and the "there is infinite work forever!" thing is most prevalent in fields like engineering, design, art, fabrication, etc.
Meanwhile, most of the people in your average office aren't doing any of those things. To some extent, they're on retainer, waiting for their skills to be needed. And, trying to optimize this is perilous. If you get employee utilization up to 100%, people complain loudly (Amazon fulfillment center workers aren't sending 1000 Slack messages a day). If you try to not pay people for the time periods where they're not being utilized, you just get the "gig economy" which is awful.
I dunno, it all makes very little sense to me. Sometimes I wonder what percentage of the US economy is about doing work that doesn't need to be done, and how many people would not have jobs if we decided "we're not paying for this anymore". I think I'm scared about it, though.