Application filtering Datacenter firewall

Dears,

UPtill today I have enabled only firewalling on the DC firewall, what make sense on enabling Application filtering on the DC firewall instead, Hence we are enabling the ports and application in the security policies apart from that  what filtering I have to enabled on the DC firewall, hence it's sound strange for me,

may be I am not aware of security insights thoughts please help me to understand.

thanks