Your estimated time of arrival (ETA) is important. Not only do you want to know approximately when you'll get there, but so do others. If you file a flight plan (and you should), it will indicate your ETA. If you don't arrive within a couple of hours of your reported ETA the FAA will start calling around to find out where you are. If they still can't find you, the FAA will send out the Civil Air Patrol (CAP) to look. Of course, at any time you can "close" a flight plan by contacting ATC (air traffic control) by radio or phone and asking them to do so for you.
To know when you'll get to your destination you not only must know your airspeed, you also must know your ground speed. The difference depends on winds and whether they are making you fly faster or slower than indicated airspeed (IAS). You'll estimate your ETA using a flight computer. This is the dead-reckoning part of the flight preparation.
Ground speed is the true airspeed (TAS) plus or minus the effect of the wind speed.