Why scientists care
At constant temperature and pressure, Gibbs energy provides a criterion for the direction of spontaneous change in a closed system under the usual constraints.
A negative ΔG indicates thermodynamic favourability for the specified change, but says nothing by itself about how fast it will happen.
A concrete example
At constant temperature and pressure, ΔG combines enthalpy and entropy to indicate thermodynamic spontaneity.
Try to notice
Science becomes memorable when a definition is connected to something measurable or observable.