Hi. First of all, welcome. It's probably because someone behind Voyager liked the idea of those nacelles, but didn't work on other trek teams so nobody bothered to come up with a starfleet ship with those nacelles.
I'm no expert on this, but I believe this would put them at relativistic speeds especially approach light speed..where time would pass slower inside the ship than outside. I don't believe they've done a story about this.
It never shows up. I'm not sure if it has ever been mentioned on screen. As far as I know, it has been included in several tech manuals, showing some kind of craft attached to the saucer section of the D or Voyager, labelled as "Captain's yacht".