actually it's the other way around: the ActiveGate is an optional component for Dynatrace that acts sort of like the Collector acts in AppMon (which is mandatory). So it's main function is being a proxy for OneAgent traffic, compressing it and making it easier to import data from different networks where you would only need one firewall rule instead of one for every server. It also has other functions as you will find in the documentation: https://www.dynatrace.com/support/help/deploy-dyna...
As I said, ActiveGates are optional. You can directly send data from your OneAgents to your Dynatrace instance.
To perform data
compression before sending to Dynatrace Managed Node
To store memory
with VMware and run remote plugins
implementations with external data: synthetic testing, mobile app monitoring,