Ben Thompson* (temporary paywall) makes an excellent first-order analysis of Elon Musk's bid to acquimerge SolarCity with Tesla. But he, uncharacteristically, stops short of seeing the mid- and long-term reasons for the acquimerge.
It's about SpaceX.
It's about Mars.
It's about the Moon.
Musk knows that he needs an incredibly-solid pipeline of technology to get SpaceX past its initial "toy" phases of being a launch company to the ISS.
He wants to ensure that he's able to support the future on non-terrestrial bodies – lunar missions, Mars missions, long-term space exploration, high-altitude space stations, etc.
Sure, it happens to be good for Tesla (integrating solar tech at Tesla charging stations is a no-brainer). But that's not the end game.
The goal is space.
* Follow Ben on Twitter – @benthompson