In the early stages when reading books are shorter, it is easier to ask pupils to read and re-read the same book several times. This is important for developing speed, fluency and intonation when reading.
These are essential skills for comprehension. Unless a person is reading at around 100+ words per minute, it is going to be difficult for them to retain the words read in a sentence long enough to understand the meaning of the sentence.
We do not need to get hung up on words per minute counts because other factors also come into the mix. But it is important to give our developing readers the opportunity to practice their reading to skills to a level of fluency that helps them to read with ease, expression and thus understanding.