Fourier series and Fourier transforms have numerous applications across various fields. They are essential in mp3 encoding, electronics, and quantum mechanics. Additionally, Fourier series are crucial in solving partial differential equations such as the wave equation and diffusion equation. Truncated Fourier series provide optimal approximations of functions over intervals using sine and cosine basis vectors. Lord Kelvin famously utilized Fourier transforms to estimate the Earth's age, demonstrating their significance in scientific calculations.