The 4th Amendment protects against unreasonable searches and seizures. The framers of the Constitution wanted government not to be able to search and/or seize places and persons without a warrant except in certain limited exceptions that were developed throughout case law over time.
Searches and Seizures and Miranda Rights
Likewise, back in the 70s in Arizona a man named Miranda was arrested and interrogated. The Miranda Warnings came out of that case and determined that citizens cannot be interrogated after they are in custody without being read and waiving their Miranda Rights. Fundamental amongst those rights are the right to remain silent and the right to have an attorney present.
If someone is arrested without a warrant then an evaluation of their 4th Amendment and Miranda Rights needs to be done to determine if there is a way to have the charges dropped.