The benefit would be the manufacturing expertise they would gain. I believe he’s on the record saying mass production is a much harder problem than the type of technical challenges with building rockets.
I used to program robots that made body panels. I think people get fixated on the “electric vs. combustion” part and can lose sight of how much other manufacturing is involved. The traditional auto companies have decades of lessons learned related to quality. To their credit, Tesla seems to learn fast but has had a lot of growing pains related to areas like quality control. There’s a surprisingly large amount of issues to prepare for when you make a seemingly straightforward design change like changing the material composition of an component to save money or weight, for instance.