Because the equation helps describe the relationship between waves and energy.
Max Planck
energy (e) equals mass (m) times the square of the speed of light (c2)
Yes. They are spelled the same too.
e, or Euler's number, is roughly equal to 2.718. For more information, see the related link. e is also used to refer to other properties. In the following famous equation, e refers to energy: enegy=...
E=MC2