Need is a source for expectancies.
Sailors in the days of schooners when it took months at sea to get any where, would develop a strong need to see girls.
You can imagine how they might have viewed some fairly large seals on rocks from a distance, and have thought they had discovered some lovely mermaids.
I suspect that more than one sailor has leaped overboard only to find that his perceptions were sorely incorrect.
A child may have a strong need to go Disney Land to see Mickey Mouse, but his mother may say, “Not to day dear…” The child may nevertheless hear what he needs to hear, “right away dear…” and be overjoyed to the puzzlement of his parents.
On the other hand, needs can, and more often do, significantly sharpen our expectancies and hence, our perceptual skills.