The main purpose of Android antivirus apps should be to protect you from catching something that could exploit your device, although a recent test of Android antivirus apps in January 2019 has found this not to be the case for around 40% of those tested.
It seems to be the aim of some to generate easy revenue for the developers, instead of protecting users stated the report.
Rather than hoping the malware protection you get after downloading the app is good enough, the test by AV Comparatives aims to help Android users to “distinguish between genuine, effective Android antivirus apps” and those that are, “dubious/ineffective ones”. It’s great to clearly see effectiveness of antimalware Android programs, for a full list and results of the said test take a look at the AV Comparatives detailed report.
The main issue seems to be some Android antivirus apps are just using app whitelists or blacklists, rather than scanning app code. You need to make sure the app you use is more thorough.