VPN on iMac

No, most of them will just cause network connection issues and macOS update/upgrade problems.

FWIW macOS has it' own built-in Security and does not benefit from AV-ware, Protectors, Cleaners, VPN's or any other third-party junk-ware. see > macOS - Security - Apple

Unless you're using a true VPN tunnel, such as between you and your employer, school or bank's servers, they are useless from a privacy standpoint.  Read these two articles: Public VPN's are anything but private and Former Malware Distributor Kape Technologies Now Owns ExpressVPN, CyberGhost, Private Internet Access, Zenmate, and a Collection of VPN “Review” Websites. 

But most are like Willie Sutton, they go where the money is. An average person - with no enemies - mainly just has to worry about phishing and scam emails, which a bit of common sense will protect you from.

Yes, if your employer requires it. Disconnect when not using i.e. for upgrades etc., should be fine.

I only use a VPN on "my" MacBookPro because my company requires it to access company resources. It acts like a tunnel between my computer and the company. That is the only valid reason to use a VPN.

Ok then @den.thed’s covered it for ya.