Notes was fun to use if your experience prior to it was Profs/OfficeVision. I had the odd privilege of using it inside IBM in the months before IBM bought Lotus, including writing the worst hack of sendmail.cf macros I thought possible to interchange between Notes, IBM’s RSCS networks, and the internet gateway we had. IBM had been developing a variety of GUI email products, mostly for OS/2, a little bit for Windows, but all in conflict with each other as well as the various OfficeVision flavors for OS/400 and VM/CMS. DEC had their enterprise email for their platforms, there was something for Novell. Notes gave IBM a single unifying platform for email and document sharing / management both internally as well as a product to sell to customers 10,000 seats at a time. In parallel the Internet went from being a thing a very small number of people were using to being the backbone of business, making a lot of the novelty of Notes obsolete. IBM in turn gave Notes the hug of death, loading up so much enterprisiness that Notes 5 felt like bloatware (it had a web server built into each client. And a JVM IIRC).
Things you got with Notes out of the box with minimal systems configuration effort: easy document sharing, easy authentication, entitlement, encryption of documents (and emails) across an organization. Easy provisioning of strong encryption across an organization. I could send digitally signed documents both within IBM and to peers outside IBM fairly easily. I could create a library of security processes for IBM's internet operations, pick out the organizations and people who could have access across the company, and it just worked. For bonus points we got document retention, audit logs, it all just was there out of the box. I didn't need to know ahead of time if someone was using Windows or OS/2 or a Mac.
It's easy to dismiss it all today as horrible bloatware, and as "just" an email tool it was probably overkill. I think too many companies (IBM especially) bought it, set it up, and utterly failed at educating people how to use it (IBMers were notorious for sending around email chains with multiple copies of Freelance or Powerpoint presentations, instead of a link to the presentation in a central repository, partly because IBM's services arm and IBM's CIO didn't want the overhead of managing such things.).