Nuclear Energy is measured in MeV, why not in Joules? Why it is preffered to measure Nuclear Energy in MeV insted of Joules?
 A: Joule is a very large unit of energy when it comes to particle physics. The rest energy of an electron is approximately $10^{-13}$ J. Dealing with any masses and energies in Joule would require consistently dealing with tiny numbers.
That might bring up the next question, then why don't we measure energies in femtojoules and picojoules? This is because electron volt is a very practical unit as well. If you are performing an accelerator experiment, for example, and take an electron or proton and take it across a given potential difference, you already know the energy it has acquired in eV. Since it is very natural to work with multiples of electronic charges and potential differences of volts (and kilovolts and megavolts...) eV becomes a very handy unit.
This is very similar to the scenario in astronomy. We often specify distances in astronomical units and parsecs because they are the right size and directly related to various measurement techniques.
A: It's simply a matter of convenience. Both are units of energy, but one is much smaller than the other (1 MeV = 1.6 x 10^-13 J). Rather than describing the energy as a very small number of Joules, it's easier to describe it as a reasonable number of MeV. It's the same reason why we use the angstrom to describe very small distances - it's not that you couldn't do it with meters, but it saves on the scientific notation!
A: Useful macroscopic energies are in joules, joules/sec which equals watts, and more. Nuclear power plants producing electricity have outputs in the range of Gigawatts, or as used in the industry, a GW. Useful particle mixroscopicenergies are evMev and GeV, as the other answers made clearly 
