Internal Junction B
The junction box SM-JC41 is used inside the frame, for internal wire routing. Even though the unit is relatively small, not all older frames will accommodate an internal junction B.
Alternatively you can use either the SM-BTC1 battery mount, external junction box SM-JC40, or not use a junction B at all.
Time trial bikes
Because you can have multiple of these junctions and they're pretty small they're popular junctions in Time Trial base bars. They are an easy way to increase the amount of ports at the front of the bike, allowing you to connect all of those time trial shifting switches.
The unit is 30.5mm long and 14.5mm wide and has a height of 8mm. If you're thinking of tucking the unit away in a tight spot, keep in mind that when installed it'll have E-Tube wires plugged into it - it'll therefore be slightly 'longer' than the stated 30mm.