1. Fuel conservation: To tow around big engine-less hangars with warships inside.
But as it was mentioned in other thread - I could just throw economical engines in such hangars and use those as slow carriers.
It's justified if you put a long maintenance life on the hangar and/or leave them in orbit of colonies with maintenance facilities for prolonged service. Think of them like orbital versions of PDC Hangars that you can move around depending on where the need for fighter support is.
With space train program failure it becomes too boring to tow such ships one at a time, so I would need not so small tug fleet to tow things around (imagine towing 300 terraformer ships from Luna to Mars with only three tugs.) So it looks like I would be better off with throwing a small economical engine in each ship.
Why not build terraforming stations with 50 terraforming modues each instead then? Civilian shipyards are not so expensive to expand.
You could even imagine these stations are actually 50 separate modules chained together that are towed by your nice space train engine at once