what is the primary use of slants? Of deeps? Of broths?

Answer 1

While you can prepare your own media, a considerable amount of cost and time can be saved by purchasing prepared media provided you limit the purchase of such media to what is actually needed. Even if you prepare your own media, do not prepare more than what is needed. Microbiological media, especially agar plates, have poor shelf life as they not only dry out but also tend to get contaminated with repeated handling.
For these reasons, this catalogue lists only those media which are essential for teaching a comprehensive course in microbiology. Prepared media are supplied, sterile, in tubes as slants, deeps, and broths and in standard size Petri plates as agar plates.