Parts of a lot of Russian weapons are of Western origin. That is stuff designed for civilian use, but can be used in weapons as well. For example GDS chips are used in smartphones, but also can be used in drones. That is the kind of stuff Russia needs hard money for. Then you have machining tools and similar products as well, which can be used to make weapons.
At the same time it enables Russia to focus its production on military products, if it can use hard currency for civilan goods.