Do I need an antivirus on a Mac?

macOS already runs one. XProtect checks every app you launch against Apple's signatures, updates itself in the background, and costs nothing.

The three things already running

XProtect — signature-based malware detection, checked on every launch, with signatures Apple pushes out separately from system updates.
Gatekeeper — refuses to run software that is not signed and notarised, unless you explicitly allow it.
XProtect Remediator — periodic scans that remove families Apple already knows about.

Checking it yourself

# is Gatekeeper on?
spctl --status

# when did the malware signatures last change?
system_profiler SPInstallHistoryDataType | grep -A 3 XProtect

Do not judge it by the bundle's modification date. That date moves for reasons that have nothing to do with the signatures, which is why the install history is the honest source — it says when the update was actually installed.

When a third-party scanner is still worth having

XProtect knows what Apple has seen. It is signature-based, so a genuinely new family can run for a while before it is covered, and it does not scan files you have been sent for the benefit of somebody else — a Windows macro you forward on, for instance. If you handle other people's files professionally, a real scanner with a real feed is a reasonable purchase.

What is not reasonable is a cleaning app offering a scan as a feature. A scanner is only as good as the feed behind it, and a feed costs money every month, for ever. A one-time purchase cannot fund one — and a scanner whose definitions quietly stopped updating is worse than none, because it still reports that it found nothing.

Where the app comes in

RefreshYourMac has no scanner and does not pretend to. It reads out what your Mac already has — the XProtect version, the date it was really installed, and whether Gatekeeper is on — so you can see the protection you are relying on instead of being sold a second one.