i found it useful in three ways.
1) If you forget to start the clocks running on takeoff (rare for both pilots to miss it, but it does happen)
2) Sometimes the F/O's and Captains clocks are set to different times only out by one minute max, but atleast you then have a definitive answer of when exactly you took off.
3) Whilst UK controllers are very very good, this gives me that little extra confidence (a completely personal thing) that they are completely on the ball.