Ok, I'll bite. I see the supposed* advantage for Solar City ("free/low" cost storefronts), but what's in it for the car side of the transaction?
Full disclosure, *I see no synergy whatsoever, and recently sold my TLSA at a loss. Still patiently awaiting my M3 number to come up, however.
Benefit for Tesla side (which has both cars and energy storage):
1. Solar panel buyers will hear about Tesla cars and energy storage in details, free advertisement for Tesla products.
2. Currently selling Tesla's energy storage is tricky. Tesla has to bid against other suppliers, negotiate details with Solar installers. It will become straight forward in the future. No bidding, simpler for Tesla's energy unit.
3. If something goes wrong with the system, is it the panels or battery storage? Going forward, just one contact person. Simplified for both sides, which means overall less work for Tesla.
4. When there are two companies integrating one system, how to decided the protocols? Especially if they want to integrate micro inverter with panels? There is no such problem within one company.
5. Solar-inverter-battery-charger-EV is really one system. Now Tesla can push hard to have one perfect system.
6. Tesla itself will need huge amount of Solar panels for Gigafactory1 and SuperChargers. Now they can produce and install their own high efficiency panels, it will reduce cost.
7. SolarCity will be cash flow positive within 2 quarters. Long term it will bring large profit for Tesla.
8. The biggest benefit for Tesla, the whole master plan would be incomplete without solar. Now it's one stop shopping, one contact, one service, which will create a better user experience, it's durable competitive advantage.