Juniper Exam JN0-231 Topic 5 Question 45 Discussion

This is necessary to ensure that the application firewall can properly identify the application and the correct security policies can be applied before allowing any traffic to pass through.

If the first packet was allowed to pass without first being identified, then the application firewall would not know which security policies to apply - and this could potentially lead to security vulnerabilities or breaches. So it's important that the first packet is held until the application is identified.