The bumping serves two purposes.

The most important one is that it creates a controlled environment where the gankers can delay and monitor the CONCORD response. You shoot the target once as he exits gate cloak to give him a PvP timer, which ensures that the ship will stay in space for another 15 minutes, no matter what, so logging off no longer saves the victim. This is obviously a criminal act so CONCORD shows up and kills the flagging alt. To counter this, you take advantage of the 15 minute timer to use a neutral alt (or two) to bump the victim at last 150km away from where CONCORD is sitting. The bumping both ensures that the victim can't just warp off willy-nilly, and that the victim is out of reach from immediate CONCORD response.

Being this far away causes the CONCORD mechanics to consider the target (and, more importantly, the awaiting gankers) GÇ£out of rangeGÇ¥ for the purpose of responding to their actions, which in turn yields the same effect as delaying CONCORD by spawning them somewhere else in the system. When responding to a crime that's this far away, the CONCORD ships first have to despawn from the first crime scene before they can show up at a new one, which delays the response by half a dozen seconds or so. You sacrifice the loss of a newbship with civvy guns for being able to execute the gank with maybe 20GÇô50% fewer actual attack ships. You can also keep a close eye on CONCORD while doing all of this, which means you have more control over the timers.

The second benefit is that the gank now happens maybe 200km off the gate, rather than 15km away from it. As a result, loot thieves will not get as much of a chance to get to the goods, and white knights stand less of a chance counter-killing the looting ships (which will go suspect in the process). If it's a freighter gank, you're likely to need a freighter to loot the wreck, and you definitely want to keep those away from the normal traffic lanes when they go blinky.

Freighters need a nice big HP buff or some mid lows. Having a 2 billion isk ship with its hold unable to haul more than a billion or so because of close to no risk / no skill ganks is pretty awful design.