There is no valid reason for this; so far on the TES forum, where I have put the question to head teachers about why they destroy the children's workbooks rather than give them out, none have come forward. The only replies I've had are from a TA who has to do it but disagrees with it, and a head who does not have a problem with letting the children take their books home. Please ask your child's school's head why they weren't given the option of keeping their books if your child did not bring them home. Chances are you won't get a straight answer and they'll hope you forget about it.
The school I work at leaves it up to the individual teacher to decide whether to send books. My job share prefers not to, so we don't. We do send home books made up from their topic work, and they have a special writing book which has 4-5 pieces of writing in per year, which we save to send home in Year 6. My sons bring their books home and I always find it a hassle to know what to do with them, as I feel mean throwing them away, but we really don't have the storage space to keep years worth of books.