For most CFW users, because it installs natively to the NAND/sd card and boots from the home menu. For PC emulation (Ryujinx/Yuzu), both work, but XCI is often easier to "boot and go" without installing.
When searching for “ Super Mario RPG Switch NSP XCI ”, you are entering the world of Nintendo Switch backup loading. This requires a hacked Switch (via modchip or early unpatched units) running Atmosphere or SX OS.