as Peterleinchen already pointed out, storageusage provides a lot of storage information right from the desktop.
apart from an initial screen with all mounted partitions (i myself have three on my µSD ) with available space you can click down into any directory down to file level
furthermore it also can display space usage by application with indication of what space is in rootfs and what is optified...

but as peterleinchen said, great to have a cli tool showing that kind of information (in a more user friendly manner then du -sm ¦-)