leakage current is like leakage of water through a small hole in the pipe which is a flow that is not needed. Amount of water leaked may be negligible compared to total useful flow.....Unless the leakage is prevented, it may damage the pipe in the future......
In transistor or a p-n junction, leakage current is generated by the minority carriers that move under the effect of applied reverse bias field....can also be generated by thermal agitation, which again is enhanced by the reverse field ...the regenerative process finally leads to the permanent damage of the junction.....