The development of X-ray free-electron lasers (XFELs) has launched a new era in X-ray science by providing ultrafast coherent X-ray pulses with a peak brightness that is approximately one billion times higher than previous X-ray sources. This dramatic advance in capabilities has already had significant impact across broad areas of science including atomic, molecular, and optical science; chemistry; condensed-matter physics; matter in extreme conditions; and biology. This paper presents some representative highlights from the Linac Coherent Light Source (LCLS) and outlines the science motivation for upgrades providing ultrafast coherent X-ray pulses at high repetition rates up to 1 MHz in the soft X-ray range (LCLS-II) and in the hard X-ray range (LCLS-II-HE).