Here's a perfect video that shows what happens with roots penetrate a sewer line.

As a business owner, and will multiple projects occuring simultaneously, I'm always inquiring of the status of projects.  I'm always asking our plumbers, "when will you be done?"  Often the reply sounds a little like, "I'm all done, except for wrapping up."  I don't even know wha that means.  But, the video proves my point.