Restorers only add Defense Power (Known as SD, or soft defense).
Basically, it represents a percentage. 900=90%.
The percent dictates how much damage is sent into burn damage and lost slowly rather than instantly.
Say you had 900 SD and you get hit by a 3000 damage shell. 2700 (90%) is going to be burnt off and 300 will be taken instantly. It also changes how much permanent (red arrow) damage is done. In the previous scenario, rather than losing a permanent 3000 DP, you only lose a permanent 300 DP. This means that you can repair to higher DPs later.
This gives your repairers a longer time to repair your ship. And yes, you are correct, once you get past 900, restorers become lame repairers. BB crews reach 900 without a restorer which is why most run without one.