This two-cell AAA battery holder built specifically for the BBC micro:bit it includes an on off switch and JST-PH connector for direct connection to the micro:bit. 
The connector mates the JST socket on the micro:bit perfectly, allowing you to easily make your project battery powered. 
If you have a battery pack with the JST connector, you know they are not suited for constant and repeated removal and insertions and frequently break.  The on off switch reduces the need to constantly remove the battery pack and extends the life of the battery pack.