The pair use the portable PC to hack from hotel rooms and phone boxes once they suspect their apartment is no longer safe.
This machine was not fitted with batteries, so its use in the phone box is suspect.
Year of feature (shown above)
I think this deserves more points for realism... an article in "Happy Computer 3/1988 mentions an "Edison adapter" as part of a hacker's toolset to get 220V current from the phone booth's light bulb socket, so they actually could have used the battery-less portable. Plus, if you look closely you'll notice the SDInet form letter (as described in the Cuckoo's Egg) on the screen during the phone booth scene.