OP, I'm a newbie here but if I may:
Water binds to the protein structure and fibers of the hair and therefore, in the case of natural hair especially, moisture is extremely important. Curly hair types are generally more porous and can soak up and retain moisture (and anything else), hence the tendency of the curly hair types to shrink when faced with humidity.
Outside factors (ie. styling methods, chlorine, chemicals/color) are what leech moisture from hair and place us in a position to have to ensure that we replace the moisture. Of course, besides lifestyle, the degree of curliness will also affect moisturization needs.
The problem is that dry hair does not have the elasticity that moist hair has. Therein lies the danger of skipping moisturization altogether, especially if you like to style your hair. Poor elasticity is the perfect set-up for hair breakage.
To answer your most recent question, since curly hair tends to be porous, whatever you are putting in it, in addition to oils is also being absorbed and that could be part of the issue with why your hair is not keeping its luster from one day to the next.