That's to clean and deodorize the bowl. However if you have brown hardened calcium deposits in the base of the bowl you need vinegar or other acid to dissolve it, because it's calcium that has formed when salt water and urine mix.

Vinegar is less dangerous than other acids and IMHO it's your preferred option especially if you have a metal holding tank. If you have a poly tank then you can dissolve it in a few minutes by using On and Off hull cleaner (barnacle dissolver) or oxalic acid diluted about 50%. That's dangerous stuff to handle though, so wear protective eye wear and gloves -- and keep it off your clothes.