They tell you everything you need to know about how long the flight was supposed to be, and how long you actually flew. Most companies even do the math for you and tell you on each flight how many minutes less than scheduled, or more than scheduled flight time the flight took. The printouts also show you the totals from each trip and for the whole month if you are terrible at difficult things like adding numbers.

The only thing the printouts will not tell you is instrument approaches, landings, and instrument flight time (there is not a logbook on the planet that is accurate on instrument flight time anyway). Also most airlines will not tell you which tail number aircraft you flew.