Water expands when heated. But water is not compressible.
The pressure increase caused by heating water in a closed tank can be quite large.
In years past, water heaters were installed on "open" plumbing systems which allowed the increased pressure to push back into the public water system.
To reduce pollution, most plumbing systems are now "closed." Today, when cold water in your water heater is heated and expands, it dramatically increases the pressure in your home's pipes which can damage your home's plumbing system.
Thermal expansion happens in your car, too. The water in your car's engine expands when heated. The expanded water is collected in an expansion tank or reservoir and is held until the engine cools down.
The information on this web site is supplemental to the printed instructions that came with your water heater. To reduce the risk of property damage, serious injury or death, read and follow all labels on the water heater and the safety instructions in the printed owner's manual.