Energy Crystals have enough energy to power two to four laser shots, a small catamaran or personal drilling vehicle, a hovercraft, and any building (all buildings requiring power draw one crystal). Bigger and heavier vehicles need more crystals. My estimation is that crystals have roughly the power output of a very small car engine. I don't know what an estimated output in watt-hours would be based on that, but it seems reasonable. Now, can 2000 car engines power a starship that's around 1000 studs long, let alone an FTL drive? Probably not, but maybe turning crystals into juice seen in the ending videos makes a difference, or maybe they had close to enough already but needed a little more.
Or maybe they needed to repair the frosting ship after it was hit by an asteroid, had most of its electrical systems shorted, and sent through a wormhole along with other space junk that most likely battered it during that trip. So while they waited for the repair crews, the raiders collected crystals because what else were they going to do?