The `splash` (which eventually ends up in your `/boot/grub/grub.cfg` ) causes the splash screen to be shown.
At the same time you want the boot process to be `quiet`, as otherwise all kinds of messages would disrupt that splash screen.
Although specified in GRUB these are kernel parameters influencing the loading of the kernel or its modules, not something that changes GRUB behaviour. The significant part from `GRUB_CMDLINE_LINUX_DEFAULT` is `CMDLINE_LINUX`