A rendering of DARPA's proposed tender satellite, in the process of removing the antenna from a defunct satellite
Satellites are very expensive to put into orbit. This is because the parts that they're built from are costly to make, but also because it requires so much energy to lift their considerable weight off the Earth's surface. It would then follow that satellites would cost less if they could use salvaged parts, and if they were lighter when lifting off from the launch pad. That's where DARPA's proposed Phoenix program comes into play. It would see a purpose-built spacecraft removing usable parts from the plethora of "dead" satellites currently in orbit, then leaving those parts for attachment to newly-arriving satellites.