Checkpoints are conducted to deter people from drinking and driving. They usually net several arrests of motorists who are intoxicated behind the wheel. In addition to this, it is common for others to have their vehicles impounded for not having a valid drivers license, registration or active car insurance policy. It is always a good idea to make sure you are conforming to the requirements required to drive in California.
The San Bernardino County Sheriffs in Rancho Cucamonga often conduct checkpoints at various locations within the city. They do not give away the exact location so that people who are drinking take another route. This way they can catch more drunk drivers and get them off of the streets. They have done a fine job in the past and I am sure they will continue to keep the city safe from dangerous motorists.