The U.S Constitution was ratified in 1789, at a time and during a period when its Framers could not have forseen all the challenges and situations that the new nation would face.
To be able to respond efficiently and effectively to these challenges, the Framers thought that it would be good to leave a door open for citizens of the country to contribute, since they were also the ones facing problems every day. Thus, it was established that the way for the citizenry to have a say in how their nation was handled through the law, was through proposed changes to the original Constitution, which are now called, ammendments.
With the ammendment process, then, now the Constitution stopped being a standard text that did not respond to the needs of the new forming country and the future generations, but a framework that allowed for all new problems to be faced as they came and for necessary changes to be made without harming what was originally planned by the Framers.