Copying from the man page:

dracut creates an initial image used by the kernel for preloading the block device modules (such as IDE, SCSI or RAID) which are needed to access the root filesystem.

dracut reads configuration files in /usr/lib/modprobe.d/ and in /etc/modprobe.d/ by default, unless run in generic mode, in which case it ignores host-specific configuration in /etc/modprobe.d/.

Again, copying from the man page:

    -H, --hostonly
        Host-Only mode: Install only what is needed for booting the local 
        host instead of a generic host.

The default configuration is host-only. This is defined in /etc/dracut.conf, in the line: