USGS states what it is not
The USGS ShakeAlert overview says ShakeAlert is not earthquake prediction. The system detects an earthquake that has already started and estimates its location, magnitude, and shaking intensity. If the earthquake becomes large enough to meet USGS alert thresholds, a ShakeAlert Message is issued. Technical partners who have a license agreement with USGS deliver an alert that can tell people to drop, cover, and hold on, or can trigger an automatic action such as slowing a train or closing a valve. This website is not one of those partners and does not send ShakeAlert messages.
Why a few seconds exist at all
USGS says the fastest waves, the P waves, travel about 3.7 miles per second and generally do not produce strong shaking. Slower S waves and surface waves, which generally do the damage, travel about 2.5 miles per second. Telecommunications are much faster than either. Someone farther from the origin can receive an alert before the strong shaking arrives. Someone at the origin cannot: the earthquake has already started there. The overview describes a sensor network on the West Coast of the United States, with nearly 1,700 sensors anticipated when the network is built out.
Example delivery thresholds
USGS gives an example of how partners deliver alerts, not a rule this site applies. Cell phone app providers and Android can deliver alerts for earthquakes of M4.5 and larger to people who could feel weak shaking, Modified Mercalli Intensity III, or greater. For moderate shaking, intensity V or greater, Android uses more urgent language. Intensity is the shaking at a place. It is explained on the magnitude versus intensity page. What to do when shaking arrives is the earthquake preparedness page.