When I used an android phone, I used the Gmail app. I was confused myself when I saw two separate apps for it, let alone one was logged in from using the Google data you entered on initial setup and the other required a separate setup.
I use inbox in android but gmail for web mail
inbox UI is much clearer than gmail and I can pin important mails and show all of them with a switch
inbox sorts emails by date (Today, yesterday). I really like it
however, the web version is just the same and doesnt have enough functions
This is quite confusing coming from Google.
The Inbox is attractive and easy, so what I thought was going to happen was eventually they (google)
would at some point adopt it as their main e-mail platform.
That never happened so I was at a loss as to what the purpose was.
It could be that it was designed to be an attractive alternative to their bland UI ?
Purely cosmetic, but if so then why not phase it in after so long ?
Protonmail is my "go to" now but I too had many questions for google about this in the past.
Using both Apps is not a problem, Inbox is like the "modern-twist" on email allowing you to easier manage task, and to an Assistant to Gmail.
How Inbox works with Gmail
Inbox was built by the Gmail team to give you a new way to stay organized and get back to what matters. Everything from Gmail (your emails, contacts, and labels) is available in Inbox. Gmail and Inbox stay in sync so what you do in one app carries over to the other. Gmail and Inbox have many of the same features, but there are differences. To get the most out of Inbox, try to use it for all your email. You can always switch to Gmail if you want to.