They do this to protect their brand. If you register for a brand, you must specify for which types of goods or services your brand should be valid. These types are categorized into so called nice classes [1], where each class has a number and covers a group of products. E.g. class 12 is for cars and class 29 is for meat products, including sausages. Your brand registration can include one or multiple nice classes, and different owners can own different nice classes for the same brand.

Volkswagen posesses a multitude of brands. First, one registered in 1956 [2] that's specific to cars, and another one registered in 1998 [3] that includes every single nice class not covered by the first brand registration.

Having a brand registered is not enough. You must actually place a product into the market, otherwise your government granted monopoly on the brand becomes invalid. They just don't want people to make Volkswagen sausages so they have to do their own.

[1]: https://www.wipo.int/classifications/nice/en/

[2]: https://register.dpma.de/DPMAregister/marke/register/621252/...

[3]: https://register.dpma.de/DPMAregister/marke/register/3980018...