Why Apple partitions the SSD drive this way, and which partition should I removed?
Solution 1:
The WDC WD100...
and the Mercury Electra
are devices. They represent the physical drives that are attached to your Mac.
Devices contain one or more volumes. Macintosh HD
is a volume on the WDC device. There will be other volumes on that device that aren't displayed.
The Mercury device uses Apple's new APFS format. This uses a structure called a Container to 'contain' the volumes. That's why there are 3 objects.
If you just want to wipe your existing volume of data, then erase Macintosh SSD
and Macintosh HD
. That will maintain any other existing volumes, such as Recovery partitions.