what is purpose or function of having email accounts on Calendar?

Here is an intro to using Calendar that might help understand how it works:

… Calendar User Guide for Mac - Apple Support

There are accounts associated with calendar because calendars are commonly stored on remote servers including iCloud, because storing the calendar stuff locally works great for just one user on just one device and not so well for that user with multiple devices or with people coordinating.

Within the app, there are also different calendars (usually remote, but can be local) that store different events. One example is the Apple holiday calendar that’s often available, and there are other servers with other calendars with other preferred lists of holidays available for those interested. And there re shared office calendars, and calendars that can be shared among members of a Family Sharing group.

I'm sorry, but I don't understand that. no one else post anything on my calendar.

I didn't say anything about anyone posting anything on your calendar. I asked where you are seeing these email accounts and suggested a possible location with explanation.

occasionally Mail asked if put something on calendar? is that what I'd lose. ok with me, since I decide and do it.

Maybe you are seeing Siri Suggestions.

Email was one of the first applications of the internet, long before the web. In those early days, it was closely associated with contacts and calendars because that is what large organizations needed. There are several different e-mail standards still in use today. Some of them include calendar support, and some have had calendar support hacked in through some clever methods. At this point, those clever ideas are now permanently baked into the fabric of the internet.

Where is it you see email accounts in Calendar?

Do you mean in the Accounts? Many email providers are not just email providers. They provide many services. Those email accounts would be the calendar services of the same account.

ArmandoWyo wrote:

but if i remove or disable accounts, what do I lose? is there a downside? some functionality that will be missing?

It all depends on what kind of accounts you have and what you mean by remove or disable.

I only keep those "ancillary email" services (contacts, calendars, reminders, notes) enabled for one account. I can still check e-mail in all of my accounts, but I don't need everything. If I were still working, then I would likely have to keep at least contacts and calendars enabled for my work account. And yes, sometimes that would be a hassle to make sure I am scheduling items using the correct calendar.

It sounds like you may have multiple calendar subscriptions for the same calendars. In Calendar, you can right click or control click on each calendar to get more information. This will tell you which account each calendar is associated with. Make sure you only have one calendar per account. If you need multiple e-mail accounts, then you may need multiple calendars.