If your goal is to use the lines for network analysis such as shortest path finding, etc, then you do not need to have M values on your line data. For network analysis, you need to make a network dataset from the lines for further analysis with the Network Analyst extension. If your goal is to do utility type work such as upstream/downstream tracing, then you need to create a geometric network and use the Utility Network Analysis toolbar.
The M values are needed for linear referencing/dynamic segmentation type work. You can use the GP tools such as Create Routes or Calibrate Routes to convert lines without M into lines with M.