We present an astrophysical science-case study of IndIGO-D, a decihertz gravitational-wave mission concept, focusing on compact binary sources across the 0.1-10 Hz band bridging LISA and next-generation ground-based interferometers. For these science projections, we consider a specific configuration in which three spacecraft fly in formation to form an L-shaped interferometer in a heliocentric orbit. The two orthogonal arms share a common vertex, providing a space-based analogue of terrestrial Michelson detectors. A central element of our study is a detailed calculation of the time-dependent antenna response of this L-shaped geometry, including the modulation produced by the orbital motion of the constellation. Assuming 1000 km arm lengths, we treat the strain-sensitivity curve supplied by the IndIGO-D collaboration as an input and investigate the astrophysical science enabled by this configuration. For the assumed sensitivity, we estimate a BNS detection rate of ~76-767 yr^-1. We compute horizon distances for different source classes and find that intermediate-mass black-hole binaries with masses 10^2-103 M_sun are detectable throughout their formation history, complementing the reach of LISA and terrestrial detectors. Binary neutron star systems are observable to a horizon redshift of z <= 0.3, allowing continuous multi-band coverage with Voyager-class interferometers from the decihertz regime to merger. A Bayesian parameter-estimation study of a GW170817-like binary shows that the sky localization area improves from ~21 deg^2 at one month, ~1 deg^2 at one week, down to 0.3 deg^2 at six hours pre-merger. These sky areas are readily tiled by wide-field time-domain telescopes such as the Rubin Observatory, whose 9.6 deg^2 field of view and r-band depth enable high-cadence, repeated coverage of GW170817-like kilonovae at this distance and beyond. IndIGO-D exploits the rapid evolution of binaries in the decihertz band to bridge the gap between millihertz and terrestrial observations, enabling early warnings on timescales from months to hours and enhancing the prospects for multi-band and multi-messenger discoveries.