I was also convinced not to need it but after having received a very nasty letter from my internet provider (caused by an old Windows notebook I was trying to bring to life and must have received an infection during my procedures) I decided to be more cautious.
True, virus are usually made for Windows OS but you may send (not being aware) an infected file to someone running Windows... or a malware can leave your mac and enter the provider's network 😱
"Adware Zap" did found and removed some minor non-dangerous nuisances (adwares) in my mac. A good small app.
However if you run a dual OS system including a Windows partition (as I do) and often move data from one OS to the other, to install a true antivirus application working in your MacOS like Eset or Symantec is IMHO not a very bad idea, although you can no doubt live without it if you are careful enough.
Curiously a well updated current Windows 10 does not necessarily need any longer additional antivirus applications as long as you leave the built-in "Defender" always in ON position, update it very often... and you do not visit dangerous web sites or download warez. Microsoft has done a good work to improve the fight with malware in their newest Windows 10 issues.
That an antivirus should make your mac run slower is often said but (without running -of course- any comparative speed workbench!) I have never noticed any such negative effect, so, if it happens, it must hardly be noticeable in normal mac environments.
Of course some antivirus applications have the bad habit to pop up at starting the mac 🙁 and nobody likes pop ups, but otherwise I can live with an antivirus... and am not afraid I could not entirely remove it one day if I wish to do so.
As mentioned, I have learned in a very unpleasant way that internet access puts the user under the authority of the web provider who can put him/her offline if the user is not careful enough. 😳
However in my painful story with my provider I have no doubt that Windows(-Virus) was the cause of my sin, not MacOS!
Ed
I usually run Windows. I'm typing this on a Win 10 system. I ensure that there is working, properly set up, antimalware software running on this, and all other, Windows systems I access. I do not _care_ if malware gets sent from a Mac or a Linux machine due to the people running those machines not having antimalware running. _They_ are not responsible for the operation of my Windows machines, _I_ am. The 'protect the Windows people' mindset is one I find particularly stupid.
Note that Windows Defender is, at best, minimal protection. I use Kaspersky on my Windows systems. YMMV.
I used to have Mac antimalware. I used John Norstrad's 'Disinfectant' for years, but that was when there actually was a serious malware threat on Macs. There is no longer such a threat, and there hasn't been such a threat for literal decades. The last serious threats were SevenDust and the AutoStart Worm, neither of which made it into 1999 before being squashed. Disinfectant was discontinued after it became clear that there was, simply, no significant threat any more.
For a while Apple included Virex in their .Mac subscription. I stopped using Virex (now, I believe, McAfee for Mac) after a beta version ate some of my email. Virex is actually more dangerous than McAfee for Windows, which is difficult to do.
I tried out Sophos; it slows the machine down, it takes a long time to scan external drives, and it places parts of itself in places where I, at least, don't think it should go. I have removed Sophos.
I tried out ClamXAV. It was slower than Sophos. The new version is no longer that slow, but it is also no longer free. I don't think enough of it to pay for it. I have removed it.
I tried out Norton. I removed it before the first day was over. It was more dangerous than many of the malware systems it was supposed to stop. Once again, the Mac version of Norton was more dangerous than the Windows version of Norton, and I nuke Norton from orbit on sight on Windows machines.
I looked at Intego. At that time they claimed to 'protect' against malware which had been dead for years, including SevenDust and nVIR, neither of which can run on OS X in the first place. I declined to allow their product anywhere near any of my Macs and haven't gone near them since. YMMV.
The vast majority of Mac malware are trojans. If you're careful what you do and where you go, you can't get hurt by a trojan. (Hint: no, that 'codec' really won't let you see that Really Great P0rn, it's a trojan and will give your Mac computer AIDS if you run it. Yes, that file really does contain naked pictures of [famous female tennis player]. It also contains malware and will infect your machine while you're busy looking at cute tennis play behinds.) There are also phishing emails. Don't click here. There are emails with web bugs embeded. Turn HTML off in your mail client and do NOT use webmail. There are adware and spyware to support adware. Use an adblocker. Certain sites (Forbes is one) hate you if you use an adblocker. I'll stop using an adblocker when they stop hosting ads which can attack my machines. As they take no responsibility whatsoever for the ads on their site, I keep using my adblockers.
The firewall built into OS X is fairly good, but can be improved if you install Little Snitch. Note that the first few weeks that you have Little Snitch installed you will be very busy approving or not approving network connections, but after that you'll have blocked most possible spyware and such.