Earlier quoted context omitted.
I find it has utility in the fact that it's a searchable record of technical conversations I have had with my colleagues where I can go to recover details I might not remember from a month ago. Then again email also does this just fine, and doesn't cost as much.
Search is one of slacks worst features and I resort to scrolling which is also majorly painful. I can count on one hand the number of times I've found what I needed via slack search in 4 years with it. I've had it be unable to find the exact text of a message on screen in a channel I'm searching in a day later (tested).
It assumes that I want to search within a certain channel or messages with a certain person when that's where I'm clicking the search bar from, but I can quickly erase that if desired. Files, messages, and people, are all searchable. It's pretty powerful and extremely user-friendly (most non-techies would have trouble with the syntax for specifying who a message should be from while searching).
Meanwhile, with Gmail, I discovered yesterday that when I'm part of a mailing list attached to an email address (let's say product@startup.com), and I search an email address that was cc'd on the product@startup.com email, nothing comes up. Resulted in a lot of wasted time.