Full codes help narrow the choice
More letters preserve more of the character's shape information. Learning the full code gives you a clearer link between the roots and the character, and often reduces the number of candidates.
A full code is not a promise of a unique result. Different characters can share codes, and the site's compatibility entries can add candidates.
When practising, learn the complete example first. You can then derive its Quick alias instead of memorizing an unrelated shortcut.