If you are just burning fuel you follow a smooth track, if you are actively tracking signals, looking between buildings etc the flight pattern will be spastic as all hell to optimize your ability to see/track/listen..
You can shove all kinds of things into a plane not typically used for signals and intelligence, with the added bonus of it not looking like an AWACS or E3...
that C5 one fascinates me. That's an awful big bird to be flying like that. (not a veteran, just grew up near Fort Knox and hubs was 6 yrs US Navy) Thought they were primarily troop/cargo transport?