According to Wikipedia, thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature. For tanks and pipes, changing dimensions after installation and the frequency of those changes can cause serious problems, with the obvious one being leaks. and pipes fabricated from FRP, thermal expansion is much lower than those made from thermoplastic, such as PVC, CPVC, polyethylene, etc.
In the graph below from Engineeringtoolbox.com (Note 1), it is easy to see how much more dimensionally stable FRP is than the thermoplastics PVC and CPVC. In fact, many people are surprised to see that the thermal expansion of FRP is extremely close to that of stainless steel.