I have a new imac. What functions REQUIRE the Cloud to operate optimally?

I think that at a minimum, you would want to use the iCloud sync feature which sync's your Contacts, Calendars, Reminders, Notes, etc. (you have control over that, of course, but having that kind of info sync'd back to iCloud can be a lifesaver if you lose or destroy your phone).

Also, you would want to use the Automatic iCloud Backup feature for your iOS devices. Again, having that data backed up in the case of of untoward event concerning your iOS device is something you would probably be forever grateful for.

The other iCloud Services - iCloud Drive and iCloud Photo Library are primarily for people who want to be able to access the data contained jn either of those repositories from any device at any time, and to be able to have full access to them via those devcies/computers. If you don't have a need for that, then you probably wouldn't need to turn on either of those features.

Here is a User Tip I wrote that kind of outlines the various aspects of iCloud and what it does for you: iCloud Deconstructed

And, you definitely, want to be signed into iCloud on your devices and computer with Find My iPhone turned on to protect your investments. In addition, you must be signed into iCloud in order to use Two-Factor Authentication for your Apple account, and you also definitely want to use that: Two-factor authentication for Apple ID - Apple Support (make sure you include more than one "Trusted Phone Number" - you can use any phone that can receive either an SMS or an actual phone call. Use your Mom's or your best friend's as a backup. You can never have too many....).

Best of luck,

GB

igiven the outrageous lapses of security of late, to hand over all of my data, photographs, written work etc. to "the cloud"

I am not aware of any security breach of cloud content by/from/with any cloud storage provider.

Any breaches I have read about are related to credential(s) data which does not directly relate to breached content. I believe any content breaches have been as a result of phishing activity mores than hacks. Users unwittingly give their credentials to, sadly, effective requests for it.

When fully documented, it seems when company A's user data list get stolen, the bad guys run the passwords they get past other services to breach them. They rely on human nature to ignore their own safety by reusing the same password or using simple to guess passwords.

The much hyped Fappening of Hollywood photo galleries a few years back was proven in court to be a phishing scheme writ large.

You had a single question in your title.

The function of having convenient access to various types of your content available on other devices is what requires one to enable and use iCloud's various services. You can certainly function without a cloud service.

You can get 5GB's of iCloud storage for free > https://support.apple.com/en-us/HT205234

No one is forcing you to use iCloud.

I've used iCloud since inception without any problems, 2011.