People write software, including antivirus software. People aren’t perfect. Ergo, your antivirus may not be perfect. Some zero-day innovative malware attack might elude your protection, and then disable it. Or you might have an existing malware infestation that prevents installation of any full-scale antivirus. Malwarebytes Free exists to wipe out attackers that get past your main defenses or that already compromised your PC before you could install protection. It can’t help with attacks that do permanent damage, such as ransomware, but it’s a handy tool when other approaches fail.
While many security companies release product updates every year, Malwarebytes only does so when necessary. The release of version 4 in 2019 was the first whole-number update since version 3 in 2016. Version 2 came out a couple years before that. The current version is 4.3.
When you install the free edition, you can choose a 14-day trial of Malwarebytes