Mantissa and exponent

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I was recently reading my computing notes and they said that to store large numbers on computer the numbers are stored in two separate parts...the MANTISSA and EXPONENT. I understand how the matissa and exponent work. What I don't understand is why computers use them because by adding in a decimal place and the exponent, aren't you just making more numbers for the computer to remember? I don't see how separating the numbers will make processing more efficient.