Imperfection in a Riks Analysis (ABAQUS)

I am performing a nonlinear post buckling using Riks method. I went through different tutorials for the same. I happen to see that an *IMPERFECTION is introduced while performing the non linear buckling analysis.Why is it done? How does/will the model react differently if the Imperfection is no introduced?

I wish to plot the graph of load v/s displacement for 1,2,3,4...10 times the critical buckling load during the post buckling analysis (Riks method). How do I do that?