Mnemonics are a pattern of letters or words formulated as an aid to memory. This is always useful because it allows a person to personalize impersonal information. This allows the person to then access memories more easily. Memory is such a fickle thing, so a tool such as this is invaluable.
However, "We need to forget if we wish to remember efficiently" (Bjork, Bjork, & Anderson). This makes sense because remembering everything would confuse us. This would force us to remember information that is not pertinent to our personal agendas. A flawless memory could easily become an encumbrance.
Russian psychologist Alexander Luria, tells of a man with such a memory who is tortured by it. He is unable to forget, and is tortured by bad memories. Also he cannot carry on a decent conversation due to his recollection of what the speaker previously said while remembering what the speaker is saying. Thinking of it in this way, a perfect memory is not so desirable.