Imagine a friend shows you a photo of a book online — well, you presume it’s a book, it could be a magazine, an encyclopedia, or even a journal, it’s hard to tell from a picture — there’s at least a lovely photograph and what could be a title, it’s hard to make out. Of course you’ve seen books and magazines, perhaps you’re an avid reader, but you’ve never seen anything exactly like this. Your friend wants you to read it before joining them in a related activity, and asks how long it would take you to read it, Minutes? Hours? Days? Weeks? Admittedly it’s not much to go on, so you decide to ask your friend some clarifying questions. How many pages? Is it a greeting card, a pop-up book, a comic book, a novel? Does it have large print? What’s it about? Who would read something like this? Your friend’s not certain, they haven’t read it, but they have heard that it’s not very long perhaps 20 pages and plenty of pictures. Sounds like a comic book, maybe even a childrens book. So, How long would it take to read it? You estimate a minute per page, but aren’t really sure so you bump it up bit. You tell your friend it’ll take about 30 minutes. It turns out, the book has small print, and a lot of technical jargon you have to look up, it’s more of an scientific journal meant for experts; in the end it takes you a few hours to read, and your friend’s event starts without you. They’re heartbroken that you missed it.
This is what it’s like estimating time for a poorly planned project. You have a goal in mind, but not enough information to know for certain how long it will take. The more pertinent information you receive the more accurate your prediction will be; and without that you’ll make an unreasonable assumption, possibly missing a deadline, and nobody wants that.