Just bought new iMac Sonoma desktop, do I still need Malwarebytes?

The best thing to do is be careful about everything you do on the internet. That includes not using torrent sites, adult sites, pirated software, software from generic s/w aggregator sites, being cautious about all email, pfishing, pop-ups, using strong passwords, etc.

But I do recommend keeping Malwarebytes. It takes up very little system resource and adds protection against a wide variety of malware, bots, spyware, malicious adware, unwanted browser add-ons and more.

You can install & use Malwarebytes for no charge if you are willing to run it manually on a regular basis. The paid version provides constant, real-time protection. Personally, I think it's worth it to know that something is actively blocking malware. I'd rather have the crap blocked before it even gets in than to have to worry about what it may have done before I detected it and then having to jump through hoops to get rid of it.

I do not recommend having any software running in the background monitoring your device. MalwareBytes is great for a free one time scan to remove Malware. You should not be paying for any subscription service that claims to offer monitored protection. The forms of Malware that you may inadvertently download will make themselves known with continued advertisements or claims that your computer is infected to get you to download additional software. When this occurs the free scan using MalwareBytes is useful to remove the Malware. After that you will be aware how you were tricked into downloading this Malware and likely will not do it again. Malware is not automatically installed just by visiting a site and needs you to actively install it.

It doesn't hurt to have it however if you follow the common sense comments MartinR mentions you should be fine without it.