Question

why is energy required for ionization potential

Answer 1

When we remove electron from neutral atom we are breaking the force of attraction between the nucleus and electron so, energy is required for this process. It is similar to breaking of bonds.

Or, you may understand it this way also. Ionization occurs when an atom loses its valence electron/s.
As we go away from the nucleus, the energy of the shells keep on increasing. Thus, to take the electron out from the shell, the energy needs to be supplied to it so that it can gain that energy to jump and then finally come out from the atom.
Thus, energy is required for ionization potentail.