Lead acid and NiCd batteries will last a long time if they are operated within the proper charge and discharge voltages. A charge controller circuit is a necessity for preventing battery over-charge. Conversely, a low voltage disconnect circuit (LVD) prevents excessive battery discharge. By using a combination of both circuits it is possible to keep the battery operating within the proper range.
Above is an extract from pdf at the link below on battery low voltage disconnect. I believe it explains the purpose of BLVDs.