It’s there as a relief valve. LAX has some of the busiest airspace in the world. ATC grants services to VFR traffic on a workload permitting basis. When ATC is too busy separating IFR traffic, which is their higher priority, it allows pilots an option that confines them to a certain area and altitudes.

For details, take a look at the Los Angeles Special Flight Rules Area[0] on the Los Angeles TAC. It gives a narrow set of exceptions. Note the specific assigned altitudes that depend on direction of travel. Also notice that the other VFR transition routes do require ATC clearance.

[0]: https://skyvector.com/?ll=33.630638921294874,-119.6291085071...