Hello!
The courts' decision that illegally seized evidence cannot be used in a trial is known as the exclusionary rule. This was decided for two main reasons:
One was to control police behavior and ensure that they only collect legal evidence with a warrant and probable cause. The second reason is that illegally obtained evidence violates a person's constitutional rights (people cannot be forced to subject themselves to searches and seizures without a legal search warrant - 4th Amendment).
I hope this helps you! Have a great day!