The Plasteel armor is the first armor type that players unlock. It is by far, the weakest and least efficient armor only used by players under level 15. Plasteel is very, weak and is only used primarily by beginner level players. Plasteel II is used for instant repair Rancor battleships.
Plasteel is the first armor in the game available to be researched. They are primarily for low levels as high low-level players tend to use Tritanium, Iridium, or Zynthium. They are incredibly easy to equip and get and is the starting block for low players on how to use armor wisely.
However, they are quickly made obsolete by Tritanium and later Iridium. It is recommended that beginner players immediately attempt to obtain Tritanium armor as it has more armor and a rather decent repair time.