Chrome, Firefox and other third-party web browsers are popular, but not universally trusted. Worse, install a third-party extension and you have to be 100% sure that it isn't malicious and hope it's not designed to extract your personal data.
Avast Secure Browser is a Chromium-based web browser (much like the forthcoming new Edge from Microsoft) which is a version heavily customised by Avast. Some might argue a little too customised. More on that later.
When you start Avast Secure Browser it looks like another version of Chrome. However, you'll notice a load of custom Avast extensions pre-installed, such as anti-fingerprinting, anti-tracking an Adblock, extension guard and much more. Some of these extensions require Avast products to be installed on your computer. You can't use the VPN without SecureVPN for example, nor can you let Avast handle your passwords without Avast Free Antivirus (or better) installed. Which means you have to rely on a lot of Avast technology to simply browse the internet.
Step up the next issue. Although Avast Secure Browser is based on Chrome, it doesn't support many of the extensions, so no themes. Worse, there's no Chrome account management either, so don't expect this Avast browser to synchronise your data.
A browser for either the most paranoid users or people so embedded in the Avast network that an Avast-only browser makes a lot of sense. We'd rather use Chrome.