The purpose of the initrd to have the initrd bootstrap to rootfs. This involves userspace tools like mdadm, lvm2, cryptsetup, zfsprogs, etc.
It's not very common to need kernel modules (drivers) on initrd on typical hardware.
It's not very common to need kernel modules (drivers) on initrd on typical hardware.
You can of course accomplish similar things by just recompiling the kernel (which Slackware makes very easy to do), but if you still need to use 'mkinitrd' anyway (perhaps because you're using LVM or softraid or LUKS), it's often more convenient to just throw in the modules you need while you're at it.