speed measured by pitot tubes is that of the wind speed relative to the aircrafts. Now, this itself depends on weather conditions which can affect wind speeds. So, then why employ pitot tubes at all if they can give deceiving readings? Can not satellite imaging be used rather for measuring accurate speeds?
There are two speeds that the pilot is concerned about. First is airspeed, and the second is ground speed.
Ground speed (speed of the aircraft relative to the ground) can and is checked by GPS. This speed tells the pilot how long the flight will be and if they are staying on track with their fuel calculations for the flight, crucial, but less critical compared to airspeed
Airspeed, (Speed of aircraft relative to the wind that is passing over the wing) is checked by the pitot tube. This speed is a more critical measurement for actual flight. The aircraft dynamics require that the air passing over the wing to be within certain ranges for different types of flight maneuvers (take off and landing have their own critical speeds, cruising flight another, stall speed is another). These can only be measured directly through a pitot tube.
Where the challenge (conceptually) is, is in understanding the difference between the two speeds. If your aircraft is flying into the wind at 60 knots (airspeed) and the wind speed is also 60 knots, then your GROUND speed, will be zero. Your aircraft is moving forward relative to the wind at the same speed as the wind is pushing it back. If you've ever seen a bird on a windy day flapping really hard, but not moving anywhere relative to you standing on the ground, this is what is going on.
Now consider the pilot turns 180 degrees and is now flying with the wind. Once the plane is at a steady flight speed, his airspeed is 60 knots, but his ground speed will be 120 knots!
Note, do not fly in 60 knot winds if your cruising speed is 60, this is ridiculously extremely insanely dangerous to do.
The speed of the aircraft relative to the surrounding air is actually much more important to a pilot than the ground speed as this is what affects the aerodynamic behaviour of the aircraft.
Consider landing, here the pilot needs to maintain a speed low enough to land safely (determined by ground speed) but high enough that the airflow over the wings and control surfaces provides enough lift to support the weight of the aircraft and control it which is determined by airspeed and this effected by head or tail winds.
Airspeed also affects fuel consumption which is an important consideration for any aircraft.
Of course knowing ground speed and position is very useful for navigation but doesn't replace the need to know airspeed as well.