Its estimated that without any form of Antivirus software you would get an infection within minutes of viewing web sites. Infections come from all sorts of places.
A web site can infect you just by you moving the mouse over an image. An email can infect you as you open it. Memory sticks, cameras and CDs can automatically run as they are inserted.
Avast Antivirus will scan all links in a web site as you open the page. It takes a fraction of a second but will protect you when you mis type a URL and end up on a spoof site. (How many times have you typed gogole instead of google?).
It will scan files and emails as you open them and stop suspicious files from being accessed. There is a “SANDBOX” facility where you can elect to open a file in a protected environment. This lets you check contents before actually saving a file for general use.
Malwarebytes is used to stop web sites from changing configuration on your machine without you knowing. It’s a different type of protection. When coupled with Antivirus, Malware protection just closes the final door on viruses.