Things in SEIV I never knew about....

[Suicide Junkie]

If your tinkering only gets your ship built one turn faster, then your efforts have been wasted; you're going to have to wait another turn for the retrofitted ship to be repaired.

Much as the others have said.
Using an example:
- Not using retrofits
Turn 2: One ship finished
Turn 4: Two ships finished
...
Turn 10: Five ships finished

- Using retrofits
Turn 1: One ship retrofitting
Turn 2: One ship finished, one ship retrofitting.
Turn 3: Two ships finished, one ship retrofitting.
...
Turn 10: Nine ships finished, one ship retrofitting.

You see the difference now?