Let me use an example to clarify:
Puer librum legendum habet
Very, very literally, this would be:
The boy has a book going to be read
This has the sense of happening in the future and passivity, and acts as a participle (as far as I can tell), but for some reason this isn't the translation. Instead, future passive participles carry a sense of necessity:
The boy has a book having to be read
Where did that come from?
I see now that some people call this a "future passive participle", but it is conventionally called a gerundive. So I wouldn't think of "going to be read" at all if I were to translate it.
A sense of prediction or obligation is inherent in any gerundive. The most literal translation is as follows—by most literal I mean the one that works in most situations, especially in a neutral context, and that is the least dependent on context in general:
Puer librum legendum habet. — "The boy has a book to read."
In English, to + infinitive can express that something will happen or should happen. Although this English construction and the gerundive are not related, they both happen to reflect this double modality: probability (will happen) and desirability (should happen). Necessity can be a kind of desirability ("this room needs to be cleaned") or a kind of probability ("random DNA mutations are necessary for cancer to develop"). I beg your forgiveness for quoting myself:
This is a complicated issue, and one that is still not fully understood by linguists, or so I believe. In short: there is a tendency in many languages for words to shift in meaning between probability and desirability. This tendency is apparently strongest in certain verbs that are used without specifying who the judge is, which includes passive verb forms.
The gerundive always has this double, shifting connotation. In some contexts one kind of modality prevails, in other contexts the other. And with dominant gerundives, such as in gerundive constructions, neither seems to prevail:
Floribus capiendis Eurydice mordetur serpente.
I'm not entirely sure, but it could be that the future passive participle simply originated as a shorthand/alternate translation for the passive periphrastic, which is defined to have a sense of necessity. In your example, Puer librum legendum habet, you could just as easily have extended it to Puer librum quem legendum est habet, meaning "The boy has a book which has to be read," and changing it to the passive periphrastic. The future passive participle seems to follow naturally as a shortening of this form.
