There are a number of reasons for flushing a borehole, however, the main reason is to purge the hole of siltation which occurs over time. Sometimes a borehole is sunk where there are high levels of fine water born particles that require extensive pumping and flushing to eliminate. When flushing, compressed air is pumped down the borehole to blow out the sands and silt that has accumulated at the bottom. Normally it is most effective.

Re casing

Re casing is most often not possible in the literal sense, but is a term used in the borehole industry to signify the placing of a second lining within an existing primary lining. A borehole requires relining when the integrity of the original lining is compromised. Over time, steel casing can corrode to a point where detritus and alluvium together with gravel packing are no longer retained and make their way into the borehole.
Sometimes a borehole which hasn’t been cased throughout can spring a leak through fractured or linings that haven’t seated properly.