The Fuschal System, in the Kaerimon Sub-Sector, was a significant source of supplies for the Warmaster's fleets during the second half of the Heresy. Fuschal itself provided huge amounts of food (mostly plankton-based) from its bounteous world-spanning ocean, and the crust of the deadly Empusa was rich in seams of adamantite and other invaluable minerals.
The Planetary Council on Fuschal promptly declared for the Warmaster when the Traitor fleet arrived to take control, to the immediate regret of the populace. However, the mining cities on Empusa, each half-underground and fortified against their planet's deadly vegetation, were of a more independent bent. The traitors' response was to make an example of one of their cities, completely obliterating the defensive walls and incendiary fields, and allowing Empusa's screaming jungle to wipe out the population inside.
With the systems population cowed and enslaved, Fuschal became a priority target for the Loyalists once the traitor fleets began their push for Terra. If the Loyalists could retake it, and destroy the garrison left behind to secure the two worlds, the Warmaster's lines of supply would be disrupted, and he might even be forced to turn around and deal with this attack from behind.