Planning gives us a lot of things:
- An explicitly-expressed understanding of what we're doing
- An explicit communication of how hard that is (points, hours, whatever)
- Accountability for how well we did (whether you call this velocity, signup, whatever)
- Insight into trends for macro planning (i.e., we get roughly X done every week, so this feature will be done in approximately Y weeks, give or take)
I'd like to make a counterargument: sometimes we shouldn't plan.
There are a lot of things that can be planned - features, data processing, even performance work. There are a lot of things, though, where planning is just guessing. Bug finding/fixing is a good example of this. "How long is it going to take you to fix all the issues we're going to find in this integration effort?" True answer: "Gosh I really don't know. Depends on what issues we find." Planning answer: "Umm... about 4 hours". With a long term concerted effort, you can figure out very rough estimates based on how many bugs you usually find in similar features and . You'll frequently be off by a bit, but statistically you can make it work out, again, with a lot of effort.
With some things and with some time periods, the effort of planning is not worth the benefit. If you can only plan with reasonable accuracy based on a lot of research and tracking, and if you don't have that, you can't plan with reasonable accuracy. Rather than just guess, consider skipping the planning.
For example, I worked with a company that was doing a big Christmas Day launch (yes, it was Christmas related). It didn't matter how good or bad it was, that was the date. We finished all the features with about two weeks to go. It was shaky, of course. If you did everything right, it worked, but we hadn't tried anything else other than the happy path. We had two weeks for testing and bug fixing and any tweaks. So we needed a planning session! Right? Nope. No way. We didn't know what bugs there were; we didn't know how long they would take to fix. Planning would have been a big guessing system. Better to get started, spend a few days learning just how brittle things were, and THEN we can plan.
Most of the time, we should plan. Most of the time we do plan. Every once in a while, we get to a point where planning doesn't make sense, and it's okay to skip it, at least for a while.