You can track if users are experiences any errors or slow page loadings.
It is real user behavior, while the availability tests scenario is recorded user behavior.
See this post on setting rules: Azure Application Insights Availability tests with Visual Studio Web Test are great for continuous monitoring, how does my system behave for specific scenarios. You also can make an adhoc web test for monitoring an experiment, feature flag implementation before you set it open for the world.
The analytics data an Azure Application Insights Availability Test produce is very useful.
You want to get alerts when on specific thresholds so you can take action before the system falls apart.