Why am I recommended to buy speaker cables with this product? It's an all-in-one stereo system - doesn't it come with what ever is necessary to set it up?

Good day I know that the hi-fi industry makes a lot of money with cables. I still bought expensive ones... I just want an honest answer, maybe with a logical reasoning. I simply use 4 cm long OFC speaker cables with 4mm2 average as a bridge. Why should these sound better for 30.-? What advantage do they have? Convince me! Thank you very much for the answer! Kind regards, Severin