This is a bush league UI problem.
Imagine, you're working on email:
You have selected a bunch of emails, some or all, in your primary inbox. They may or may not scroll out of view, but they're selected. You realize there's an email you need that was in another folder, say Promotions. You go to the Promotions folder. You click on the new email. You select it, then move it to inbox. While you're there, you want to delete the rest of the stuff in that same Promotions folder. So you select all, by hitting the button, and then, because you're a power user, you hit the keyboard shortcut for deleting it (shift 3). This takes a single second because you work quickly, but the button doesn't work. So you try again and again, wondering if there's an internet or Gmail lag, and eventually realize you're not hitting and never hit the "select all" checkbox, but you're hitting the "primary" folder ICON. So you go back to the promotions folder, remember the true location of the select all checkbox, and try to delete the contents of that folder. You're successful.
But little do you realize that you also just deleted everything you had earlier selected in the Primary folder, and your "undo" time window has long since expired.
If you need help with the UI design of Gmail, I'm just a piano teacher and a small business owner with a Bachelor's degree, but I'm sure I can help you do a better job of locating things in a more thoughtful way than has been done in this new "updated" form of the client.
If you didn't catch the shade I just threw, I'll make it plain for you:
This new layout is DANGEROUS and ILL-CONCEIVED.