VW have significant experience of producing cars for other manufacturers, and put a lot of effort into developing reusable "platforms". They are super popular in Europe- hitting the exact sweet spot between practicality and desirability (Top Gear described the Golf as "the car that everybody buys with their own money"). The forthcoming ID range is perhaps the strongest lineup of EVs in any manufacturer's pipeline. They need more traction in the US, and it therefore makes sense to partner with the arguably coolest US corporation.