Based on the direct modulation of diffraction channels by localized surface plasmon resonances from metallic nano-structure or Mie scattering from all-dielectric micro/nano-structures, a metagrating emerges as an ideal platform for high-efficiency manipulation of wide-angle diffractive optical fields. In this paper, the physics and applications behind metagrating-based manipulation of diffractive optical fields are reviewed. First, metagratings with desirable high diffraction efficiency are introduced, including different working schemes of reflection and transmission modes as well as symmetric, asymmetric, and reconfigurable configurations. Second, by incorporating the high diffraction metagrating with displacement-coded phase modulation mechanism, arbitrary wavefront shaping with a wide-angle range can be realized, leading to their special applications in high numerical aperture metalens, angle-tunable multi-functional devices, and wide-angle hologram metagratings. Third, considering the metagrating as a bridge to connect the surface wave and free-space wave, the mutual conversion between a free-space wavefront and a surface wavefront is analyzed. Finally, we give a brief summary and the development trend and the potential applications of metagratings are prospected.