Months ago, I estimated a feature to take 3 weeks to build. A couple of months later, deadline nearing, a new developer was asked how much time it would cost to build and they told the product owner that it would take about 3 weeks. The product owner then looked at me, annoyed, telling me that nobody ever told them that it would take that long to build. I was really happy that I could show them the initial estimate I gave them.

If you haven't experienced this ever, definitely be extremely grateful. Also, when you switch jobs, beware that past experiences are no guarantees.

I think that's naive. The parent comment phrases it a bit more cynically than I would have, but it's in everyone's best interests to casually document plans and intentions. People forget, people misremember, people promise things without fully thinking through the implications in the moment. A friendly email restating the key points of a discussion is conscientious, professional, and keeps everyone on the same page.

I'd personally be very suspicious of anyone who tells you not to do that. There is literally no downside unless you're trying to hide something.

As others have stated, it’s not about malice, but rather about miscommunication, misunderstanding or simply forgetfulness.

It has happened many many times that I thought I was clear on a thing and that everyone was on the same page to later realise that some of us completely misunderstood what was discussed or agreed. Sending a summary email afterwards helps make sure that everyone came away with the same conclusions.

I’ve also found it extremely common that people simply forget things, or parts of things. I’ve had bosses who forgot that they asked for a change or feature or task to be completed. I’ve had bosses who forgot that they agreed to change a requirement or that something isn’t needed or important. I’ve forgotten that I said I’d get something done. I’ve had coworkers forget that they were supposed to do certain tasks.

By sending a quick “here’s what we agreed to” email avoids all of these issues, or, at least, let’s you point at it later if someone questions it later.

Basically, it’s a good idea to keep a log of decisions that were made.

I'm probably over-selling malicious - other posters have covered the mundane reasons better.

But anytime I've suddenly been stuck feeling "wait how was I doing the wrong thing?" the overwhelming reason has been traceable to the fact I can't point to any specific bits of correspondence which justify why I was doing something a particular way because it was all ephemeral.

For me, it's a workplace confidence builder - there's not a lot of light between a manager forgetting something you said, or hearing something different, from your perspective when the question posed is "why did you do a thing?". In those moments, being able to point to specific records of correspondence means the discussion now has a concrete starting point - even if it's going to turn out you absolutely misinterpreted what was asked.

It's hard for people to have recall over events and (again not always malicious) what one person may consider a "big deal" to have done (or not done) will influence whether they remember and how they remember it. I've been pulled through changes I was opposed to (in my opinion, it made less sense to make the change) and eventually conceded to the change. 1.5 years later everybody (both those who agreed with me at the time and those who disagreed) was all about changing it again (back to the way it was). I had the email conversations and remembered what previously happened and was able to (re-)inform people about why the decision had been made (no, I did not want to revert all the work I did even if I still thought we shouldn't have made the change).

Not everybody has access to a better work environment...