I've never really seen this question anywhere, so I'm not very optimistic about a clear answer:

Why did the Death Eaters care so much about Sirius' whereabouts in OOTP?

Yes, I know Voldemort realized that the one person Harry cared most about in this world was Sirius and vice versa; Kreacher's information to the Malfoys, the plot regarding Harry and Voldemort's connection and seeing into each other's minds. HOWEVER, why did the death eaters care so much about finding him even before the Kreacher/Malfoy info dump? Multiple times in OOTP though, there are lines referring to the importance of keeping Sirius hidden in the context of the OOTP's war against the Death Eaters. Did he hold certain value to the dark side? I'd love this cleared up!