Your web and mobile applications are expected run flawlessly round the clock. To make sure they run as expected you need server monitoring tools to watch your servers and applications. Also, you need people and process in place to address alerts from these tools any time of the day or week.
Open source tools are sometimes too basic to monitor a system on which a business relies. On the other hand, they can be useful toolkits that a development team can use to build exactly the type of monitoring tool they need. Of course, that team would have to have one or more members willing to manage and maintain the monitoring system they build.