So you'd have some sort of interstellar tug to move it from system to system? 
What I'm thinking is that it takes a long time to charge the weapon, it can't be held charged for very long, and it can't be taken into hyperspace charged because <insert technobabble>. So you've got plenty of power for a hyperspace jump, but you then have to crawl around the target system putting most of your power into charging the weapon with only a little left over for propulsion, making sure that you end up in range of the target just as you hit full charge, otherwise you're going to have to blow up some random asteroid and start again.
From the scriptwriting point of view, this gives a reason for the normal-space crawl towards the target that give the defenders a shot at stopping it: one of the big unadressed questions in the original movie is why the Death Star didn't just drop out of hyperspace right next to Yavin and blow it up ten seconds later*. It also keeps the purpose of stealing the Planet Killer's plans, when this version is all weak spots: it's the escort you've got to defeat. You need it's performance specs in order to work out where it can shoot from and how long it takes to charge, which in turn lets you define the area of space that needs to be monitored in order to get enough early warning to coordinate a response.
*Off topic, but it's just struck me that this is a plot hole in Rogue One. In a New Hope, the Death Star has to travel in normal space for over 15 minutes to get into firing position on Yavin (source: Rebel command centre dialogue). However, in Rogue One, it pops out of hyperspace practically on top of Scarif and takes a shot about 1 minute later (source: timings of space battle scene). Okay it's "only" a "one-reactor" shot, but it's still pretty devastating and would more than suffice to finish off the Rebel base on Yavin. So since it worked so well at Scarif, why didn't they do the same thing a few weeks later at Yavin? Also, where was the Imperial fleet at Yavin?