Diamictite is a name favored by careful geologists because it signifies only observable matters without assigning a particular origin to the rock. Conglomerate, being made of large rounded clasts in a fine matrix, is clearly formed in water. Breccia, being made of a finer matrix bearing large jagged clasts that may even fit together, is clearly formed without water. Diamictite is something that is not clearly one or the other. It is terrigenous (formed on land) and not calcareous (that's important because limestones are well known; there's no mystery or uncertainty in a limestone). It is poorly sorted, full of clasts of every size from clay to gravel. Typical origins include glacial till (tillite) and landslide deposits, but those cannot be determined just by looking at the rock in the field. Diamictite is a non-prejudicial name for a rock whose sediments are very close to their source, whatever that is.