You don't need to, you like to. You being a stand in for the general readership.

People sign up for email newsletters, people dont subscribe to RSS feeds. They do visit websites, which is how I read the couple substack authors that I do; but if you've already got the content why not email as well?

Because my email is for work, maybe some automated notifications I haven't moved to a messaging platform yet.

Email is not a thing I use to read long-form content like newsletters. It just doesn't match my workflow at all.

When I receive an email I expect it to be something actionable, something I need to react to, not "Hey, here's some cool stuff for you to read and a bunch of links". I have Twitter, RSS and other platforms for that.

Some newsletters are purely informational. I like to update my customers on upcoming holidays because it affects how we do business. I also like to update them to remind them of where they can get our W-9 form for tax filing. They're not really the type to use RSS or check the website.

I do get the idea of email updates in traditional businesses, it's mostly marketing.

But why would heavily technical people want to clutter their inbox voluntarily with newsletters? Do some people enjoy reading long-form newsletter content in their email client that much?

That I don't understand. As has been proposed as a solution before, I see a lot more utility in a daily or weekly roundup of all newsletters delivered to my inbox. I do like to read a few specific programming-related newsletters, but don't actually clickthrough that frequently.