Why is Germany taking a big stake in a major tankmaker?

Pretty normal in Europe. Governments like to keep strategic stakes in key defence companies operating in their area. One being simply “if we own them, they can’t leave. That is good for longterm supply security.”

Finland owns control of Patria, Norway of Kongsberg, Sweden doesn’t own SAAB, however Wallenberg family is as much a swedish institution as the state itself. Airbus is pretty much dominated by ownership stakes of the various governments part of the joint program. France owns Nexter/GIAT/half of KNDS. Italy owns strategic major stake of Leonardo.

Germany highlighted the importance of working closely with France in its statement, saying the deal “aims to strengthen bilateral and European defense cooperation in particular, with collaboration with France playing a key role.”

It also emphasizes that it will work “in close dialogue and on an equal footing with its French partners.”

Or for the translation “Diplomatic” to “English’: Without an equal stake France will fuck up this (officially German-led) shared project, too, with yet another “we decide the specs, develop the tech, get the work load but you can get half the bill"-move.