Because it's the inverse of an exponential.
Why is an exponential used?
Because it's the special function whose value is equal to its slope everywhere.
Why is that useful?
Because the rate of change of quantity y is proportional to the amount y left.
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Carl E Mungan, Assoc Prof of Physics 410-293-6680 (O) -3729 (F)
Naval Academy Stop 9c, 572C Holloway Rd, Annapolis MD 21402-5002 mailto:mungan@usna.eduhttp://usna.edu/Users/physics/mungan/