Hi Madrigal,
There are different types of mattress protectors (vs mattress pads) which each have different degrees of protection against moisture (waterproof vs water resistant vs moisture absorbing), their effect on the “feel” of a mattress, and on their effect on the sleeping temperature of your mattress.
There is more about the tradeoffs involved when you are choosing between the different types of mattress protectors in post #89 here.
The St Dormeir is a very popular choice because it’s thinner than most other cotton/wool mattress protectors so it will have less effect on the “feel” of the mattress, is “water resistant” which for most people would be “enough” protection, and with the wool can also have a positive effect on temperature regulation compared to other less breathable mattress protectors.
Phoenix