Why the service with the enable status “indirect” can’t start automatically on boot?

Issue

• Why the service with the enable status “indirect” can’t start automatically on boot? For example:

systemctl status tftp.service
○ tftp.service - Tftp Server
     Loaded: loaded (/usr/lib/systemd/system/tftp.service; indirect; vendor preset: disabled) <--indirect
    Drop-In: /etc/systemd/system/tftp.service.d
             └─allow_uploads.conf
     Active: inactive (dead)        
TriggeredBy: ● tftp.socket
       Docs: man:in.tftpd