Hello - I only registered tonight and have not yet downloaded the K-9 client. My interest in K-9 is seeing Rob Braxman install this on his de-googled phones and I am interested in taking back more of my privacy. Currently I use gmail for all my personal email and I access it on my android phone and desktop computer and am looking for alternatives At first I thought K-9 was a server but it seems to be an email client for android phones only. May I presume that most people are linking gmail (or other popular email providers) with the K-9 client on their android phone? If so may I ask what is the point? If one was to link gmail with K-9 that would not actually detach oneself from gmail and Google still has full access to your email content. I don’t mean to be disrespectful in my tone as I look up to open source developers and all the work they do (i use linux desktop), just trying to learn.
It’s an email client that can fetch emails from any provider, not just Google. I myself have 2 Gmail and 2 Yahoo mails that it picks up and I’ve seen others use it with all kinds of other providers.
Not everybody wants to detach themselves from Google 
K-9 Mail is an email client that manages your emails on a server. In that regard it’s similar to a browser.
Switching from Chrome to Firefox would do little to remove your dependency on a hosted service like Google Docs. But switching to e.g. a self-hosted online office suite would.
So if your goal is to get rid of Gmail, switch to another email provider. Most of them implement the open standards IMAP and SMTP, so you can use email clients like K-9 Mail with those providers.
Bit if you want to detach from Google and self-Host your mails in your own own loud or somewhere or if you use any server you trust K9 can display your mails on your phone without the need of help from anything Google.
If k9 would also provide the server infrastructure and all of this for free I would immediately start to wonder what they pay their servers from…
Also K9 supports encryption of mails with PGP by using seamless integration with OpenKeychain (another free app). That was my main reason to start with K9 years ago. Unfortunately too few people use PGP so most of the mails I have to send are still unencrypted 
In addition to the above replies, K9 can pull and send mail to any IMAP compliant email service. I know of four that aren’t: hey.com, Onmail, ProtonMail, and Tutanota.
K9 can also handle multiple accounts. I use K9 with all of my personal accounts, accept the above four services I mentioned. It’s nice to get everything together in a single app, rather than opening up GMail, then Yahoo, then Outlook, then Fastmail… You get the idea.
HTH
Just wanted to say thank you for all the replies, now I understand.