Liliana initially watched the crisis from Vess Manor, focused on the Chain Veil and the demons controlling her contract. Innistrad’s dead were unusually resistant to Emrakul’s transformation, giving her necromancy an advantage living armies lacked. That usefulness did not make her motives suddenly simple.
At Thraben, Liliana brought an immense host of zombies against Emrakul’s changed followers. The attack pushed the titan back and kept the Gatewatch from being overwhelmed, but the Chain Veil strained her body and judgement. Power that could save a plane was still power capable of consuming its wielder.
Her intervention mattered because she chose to remain when departure was possible. The Gatewatch needed exactly the person it distrusted, and Liliana needed allies for conflicts she could not survive alone. Their cooperation began as a practical bargain before anyone could describe it as belonging.