Iteration numbers allow you to create a sequence of successively finer meshes suitable for multigrid methods. They also allow you to produce a sequence of meshes using error estimate-driven mesh refinement.
If you're not using refinement or quality meshing, and you don't like iteration numbers, use the -I switch to disable them. This switch will also disable output of .node and .poly files to prevent your input files from being overwritten. (If the input is a .poly file that contains its own points, rather than referring to a separate .node file, then a .node file will be written.)