Cosmic inflation solves the horizon problem by expanding space faster than light. This allows regions initially in contact to spread far apart. Without inflation, distant areas would have vastly different temperatures. The uniformity is evident in the CMB. This insight led to the development of inflationary cosmology. It explains why the universe looks roughly the same in all directions.
It matters because solving the horizon problem validates inflation models.
It also informs our understanding of early-universe causality.
The universe appears uniform thanks to rapid inflation after the Big Bang.
CERN [cern.ch]