Condottiere said:
Shipping still takes time, compared to local manufacturing, which can be customized to precisely fit an order.
True, but that's been the case since the beginning of trade. There are some facets of an economy where trade makes sense, and other parts where it does not. Economists would argue that nations should stop 'ruinious' trade barriers to basic foodstuffs that a nation may or may not be able to grow, and concentrate on what grows
best in their climate/soil.
But economists don't live in the real world. Growing your own food to meet your basic caloric needs is also a matter of state security, i.e. if you can be starved out, you are vulnerable to a potential enemy.
Also Traveller doesn't model the required underlying support structure necessary to build/support things. Ethiopa flies 777s and other similar countries fly 787's, some of the most advanced aircraft in the world. And they do maintenance on them, at least basic stuff. So while the can repair things, they do not have the national infrastructure either logistically, educationally or even materially, to build aircraft like that. Even shipping in components to be mated together can be challenging fora nation that has none of the underlying support structures to do so.
And that's why trade exists today, yesterday and probably in the 52nd century too. Even ST, with it's magical replicators, had trade.
So to keep on topic, maybe you have your construction fleet coming along with your invasion fleet. Your fleet exhausts it's supply of ammunition in battle. Your construction fleet says "hey, we've got rocks, we've got gas giants, we can make new ammo for everyone!". And four to six months later you have your ammunition bunkers full again.
OR you could have simply brought a few ammunition transports up from your fleet base (or your fleet train) and resupplied your war fleet in a couple of days. Meanwhile your infrastructure to support all this is safely weeks away from the front lines while your manufacturing fleet is a sweeeet target of opportunity for an enemy looking to cripple your attack.