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It's more for downloaded files, so windows defender would actively scan them as you download, but as a secondary protection, once they're downloaded and before you proceed to install or extract or something, you right click the file and scan with malwarebytes.Just looking there on MB.com - it says the free version will only clean up an already infected system after an attack. Is that basically the idea here? That Defender and careful browsing keep you safe, but you have MB in case anything gets through the net?
Or are MB telling little porkies to get you to buy the Premium version! I assume you can manually scan your system with the free MB whenever you want?
Then once you're satisfied both scanners return safe results, you can proceed.
But for a full system scan, it's just an added layer to run once a week or something. No scanner is perfect, they all are "catching up" with new infections, so having 2 just increases the chances that you'll catch something before it's too much of an issue.