Average people can hold seven chunks of information in their short-term memory at once. Mnemonics make memorization easier and more efficient by compacting wide swaths of information, that might take up many chunks of memory, into one smaller chunk. For example, while remembering the order of operations without a mnemonic requires memorizing six separate chunks of information in order: parentheses, exponents, multiplication, division, addition, subtraction, with a mnemonic, it requires learning only one phrase: "Please excuse my dear Aunt Sally."
Since images that are happy, colorful, sensory, unusual or silly stand out in memory the best, effective mnemonics incorporate vivid imagery, details and/or rhyme to make otherwise bland data meaningful -- so it will stick out in your mind. The more imaginative the mnemonic, the easier it, and the information associated with it, are to memorize. "Kids prefer cheese over fried green spinach" provides a meaningful, concrete, colorful and humorous image that kingdom, phylum, class, order, family, genus and species (the taxonomic hierarchy) on its own can't.
Mnemonics can serve as a teaching device to assist in memorizing information. For example, when working on a mathematical equation, if you remember to do parentheses first and exponents second, but can''t remember what to do next. By recalling the mnemonic, "Please excuse my dear Aunt Sally," you will know the next operation instantly even though you haven't yet mastered the actual order of operations. For this reason, repeating "Please excuse my dear Aunt Sally," will help you memorize the order of operations faster than simply repeating the actual order of operations will.
Memory is like a muscle -- the more you work it out the stronger it becomes. By strengthening the brain's ability to learn and memorize, mnemonics make learning, memorizing and accurately recalling large chunks of information easier in the future as the brain continues to develop. The more mnemonics are used, the more natural inventing them becomes to the point where people with highly developed memories can quickly memorize new pieces of information by almost reflexively associating them with other things.