There are good reasons to make assumptions early on in a project when narrowing down the solution space. You have 10,000 golf balls and you need to get them 5 miles south of here. You ask your coworker to look out the window at your company parking lot and tell you what vehicles you have at your disposal. A sedan, a double decker bus, an SUV, a small trailer, a motorcycle.
If you have reasonable estimation skills, then by the time you get out to the lot with a tape measure, you will have narrowed the search down to the sedan and the SUV. You don't need to bother measuring the double decker bus because you know that it's around 100 times as large as needed. You don't need to consider using the motorcycle because that is clearly far too small. If you collected data on those options, you'd be wasting time, because in the time it takes to collect the data, you could have already ruled them out.
"OK," you say, "but it doesn't hurt to be thorough." But see, it does hurt to be too thorough. Every second you waste on deciding not to use the bus is a second you could have spent making a better decision between the sedan and the SUV.
But it doesn't matter what the aim is. The point is that estimation based on very little data is important because real world situations call for narrowing down the solution space quickly.
If you have reasonable estimation skills, then by the time you get out to the lot with a tape measure, you will have narrowed the search down to the sedan and the SUV. You don't need to bother measuring the double decker bus because you know that it's around 100 times as large as needed. You don't need to consider using the motorcycle because that is clearly far too small. If you collected data on those options, you'd be wasting time, because in the time it takes to collect the data, you could have already ruled them out.
"OK," you say, "but it doesn't hurt to be thorough." But see, it does hurt to be too thorough. Every second you waste on deciding not to use the bus is a second you could have spent making a better decision between the sedan and the SUV.