New designs for airplanes are brought forth all the time, and quite a few pass the muster, making them candidates for further interest by the military, privately and publicly owned airplane manufacturers, and commercial airlines. When a new design gets passed through to the design and then the manufacturing process, well, someone has to "try it out", sort of like taking the new Corvette Anniversary Edition, first-run, first off the assembly line beast for a test drive, putting it through the extremes of acceleration and driving. The same, but totally different, is the new airplane's flight test process.
Take, for example, the F-35 Joint Strike Fighter, which is reinforcing the American superiority in the dog-fighting aspect of air-to-air combat, as well as bombing runs and ground troop cover. This supersonic jet can scramble from 2,000 miles away, and bomb the bejesus out of the man in the boat on a Canadian dime, It is the best, fastest and most acrobatic jet ever designed, and the test pilots have to put the jet through it's most arduous of abilities. Being a test pilot used to be one of the most dangerous jobs around, but with advancements in the ejector seats, and with the use of simulators, the pilots can now escape unharmed when the planes fail. But the onus is on saving the plane, pilot and anyone else who may be in harms way if the plane were to crash.
If a plane were to be allowed to carry passengers, or to engage in a military operation, without first going through a vigorous flight test process, it would open up the manufacturer, designer and/or carrier to major lawsuits, mainly for not ensuring that the aircraft is air worthy. The flight test process works on all probable and possible failures within the airplane's systems, so that if the same conditions occur in flight with passengers, or during a military operation, then the correcting actions are known to the pilot.
Flight testing is done to ensure that the manufactured airplane meets the standards that were laid out in the Request For Proposal (PFP), the initial order for the airplanes by the carrier, which lays out exactly what the aircraft's limitations, size, speed and accommodations are. The initial portion of the flight testing procedure is for the pilots to train on simulators, which react exactly the same way the final aircraft will with a view to the limitations of the aircraft, and the correcting actions for any possible and probable malfunctions that can and do occur during takeoff, flight and approach (landing).
For automobiles, they have test drivers who put the first cars out of the manufacturing facility through stringent testing procedures. They accelerate to the maximum, and brake full on at top speed, test the turning radius and the road worthiness of the vehicle. The same has to be done for airplanes, but with much more of an eye on regaining control of an out of control aircraft, or bringing a malfunctioning airplane in for a safe landing.
When airplanes crash, there are more than just the people in the plane at risk. People on the ground are targets for aircraft that are out of control, have lost all flight systems and are heading towards populated areas. In cases like these, the pilots try to aim the plane at the least populated area possible, and try for a crash landing, at the proper attitude (plane level with regards to the earth), so that the plane would not break up upon impact.
All of the crash landing scenarios that can possibly be thought up are covered during the simulation process, so that when the test pilots are in the actual in a plane, they know exactly what to do when certain systems, or the integrity of the airplane itself is compromised.