The police must get a warrant to carry out an official search in an area where you have an expectation of privacy. With only a few exceptions such as if they think evidence is actively being destroyed, for example, they must have a warrant to search your home.
However, they may be able to collect evidence without a warrant, and without officially carrying out a search, if you don't have a right to privacy.
For example, maybe you were arrested on drug charges. You came home one day and set a package containing drugs on the ground next to your car while you carried in some other items. While you were inside, police came and took the package from your driveway.
You argue that you had a right to privacy on your own property, saying the search and seizure was illegal. The police counter by saying that while they couldn't have taken the package if you'd carried it inside, you couldn't expect a package left in the driveway to remain private.
One question, then, is how far does this extend?. What if your driveway is a mile long, running back through acres of your own property? Do you have more of an expectation of privacy than someone in the suburbs whose driveway is just a few yards long? What if the package is in your back yard? It's still outside where anyone could see it, but do you have more of an expectation of privacy there than in a driveway that can be seen from the street?
These issues can be rather complex, but it's critical to know your rights under the Fourth Amendment when you're facing drug charges.
Source: FindLaw, "Searches and Seizures: The Limitations of the Police," accessed Aug. 10, 2017