Plasteel Plates are the first base armour you unlock in the game. It's the base version of Plasteel, and many low level players will end up these Plasteel plates on their bases, due to it being their only choice despite its very low armor boosts. They should be replaced as soon as possible by Tritanium plates or superior armor.
Plasteel Plates 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.