1. You don’t waste time on estimation
Estimation takes time. Even if you do planning poker and use story points, it still takes time. What do you do to improve estimation accuracy? You gather some data, analyze the data and discuss the results. You are spending time on all that. But are you sure that estimates really provide any value? Most likely not. It is a waste. You’d better spend this time doing something important for the product.
2. You shouldn’t explain to higher managers why it took soooo loooooooong
If you don’t have estimates, you can speak without fear and explain things clearly. You can enumerate problems and explain how they were resolved. You can show that you work really hard and take all the necessary actions to release this story as soon as possible with great quality.
If you have estimates, be ready to hear something like “you screwed up, maaan! You had to deliver this feature till the end of the month! What the f%$k is going on?” as an argument. The team put themselves on a weak side immediately and have to care about deadlines more, not about quality or a better solution. Is it something you really need?
3. You don’t give promises that are hard to keep
You are not relying on people’s optimism (which is built in). People are optimists (almost all of them) and inevitably give optimistic estimates. You can use complex formulas or very simple rules to have better estimates, but is it really worth it? You can spend many hours defining correct formula for exactly this development team. People will not trust you, since you (rightfully) don’t believe in their estimates. Numbers will look “made up” and still you will have incorrect estimates in the end.
4. You don’t put additional pressure on development team
In some teams Estimate is equal to Deadline. That’s bad. What do you do to meet the estimate? Compromise quality? Tolerate average architectural solution? Abandon polishing? All that leads to poor solutions that end users will not like. It is better (almost always) to spend more time, than planned, and release something really usable and useful.
5. You focus on really important things
Imagine, you have a planning poker session and estimate all stories inside an epic. You sum up all the stories and define it as a 1000 pts epic. You just stick a “fear” label on the epic. People do fear large problems. A 1000pt problem is huge and psychologically it is harder to decide “let’s start it right now”. Product Owner will have a temptation to implement 100 smaller user stories instead of this huge epic. However, it may be a bad decision. This epic may be really important, the morst important thing for the product.
If you don’t estimate it and are not completely sure about its size, you have a better chance to actually start doing it.