Well For Freighters, travelling faster mean you can trade with further planet(s) faster, Thus more efficient in trading.
As for fighters, It should be fitted with the best engine(s), so that it can move faster.
This exploits a 'bug' in which the game is designed. If your fast fighters can move and hit enemy fighter(s) first with it weapon(s), it will have a very good chance of destroying it. For Example, your fighter with 3 beam, 10/10 move point with 16/16 hit point can move around and destroy fighter with 0 shield, 10/10 move point, 12/12 hit point, if they are in range. This does not matter if the enemy fighter has greater attack power than your fighter or not. The bottomline is to strike first!. Due to the poor sensor ranges that your fleet may have, the move point provided you the availbility to search an area for the enemy ship that you lose track of or you saw it fly past a visible area. By moving and searching and finding the enemy and give them a first hit is very important if your weapon directly hit it (ie for example, using beam to attack no shield ship). This give you a chance to destroy them, but if they found your ship before you find them, they will have the strike first advantage.
I hope GC2 can improve the ship tactics and be wary that it can become quite unfair if a fleet with better weaponary but little defenses(armor/shield) loses to attacking first ship with lousy weaponary. I know GC2 may claim that it the user fault to not design more defenses for their ship. But the lack of Ship tactics make unarmed ship totally useless against armed ship even with powerful engine(s). The ship battle also noobifies and have little control over what to attack fight in a fleet or how to go about attacking the fleet effectively with a tactic. GC2 should learn from Stars! which has very good ship battle tactics like being able to engage/disengage, choose the attacking targets, minimising demage to self or even maximising attack power.