The samples are needed because our knowledge is curtailed by the limited extent of our previous lunar missions. The six Apollo missions that returned with rocks all landed in the same region, on the central nearside hemisphere of the Moon. To make matters worse, the region was geo-chemically anomalous, and as such the samples did not reveal very much about the composition and origins of the lunar crust.
Chunks of lunar rock that split off from the surface of the moon (due to various unknown impacts) and hurtle into Earth are ideal samples since they are completely random. Scientists and professional collectors have so far found around 100 lunar meteorites - give or take a few dozen depending on whether one counts meteorites that may have been found as separate rocks, but are suspected to have originated from the same meteor event.
ANALYSIS: Russian Meteor Explosion Seen From Space
The rocks are used to confirm the observations of sophisticated spectrometers that remotely sense the mineral and elemental composition of the lunar surface.