Nrchy sez: "Usually they were left over from overpressing LPs that ended up not selling for full price."
This is true, and I know Nrchy was not trying to say this, but it does not always mean that the record is the gazillionth one produced from the master and of poorer quality. I have heard some folks equate the two and it is just wrong.
I have many outstanding recording that are notched. In the case of classical, some jazz, some off-beat rock, and especially my favorite bluegrass scores lotd of titles were produced in relatively limited numbers for a limited market. So, depending on the title, label, and genre, they can be a heck of a bargain if the seller thinks they are in some way inferior.