Why Apple partitions the SSD drive this way, and which partition should I removed?
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.