Hello.

New to the game.

What, other than a sudden stop, would this upgrade be used for. Doesn't seem like it's worth it.

It's a strategic card. Most often it gets used to keep a target in arc or set up a block.

Thank you!

In a game where most ships are constantly in motion, the ability to stop for a turn can catch your opponent out if done well.

Some builds use it to park several ships in formation by bumping the rear ships into the front one while it is stopped and forcing the enemy to come to you entirely on your own terms. This is sometimes called fortressing but is not particularly popular as you lose the element of initiative and some people regard it as unsporting.