approximating the number of distinct elements in a multiset. Calculating the exact cardinality of a multiset requires an amount of memory proportional to the cardinality, which is impractical for very large data sets. Probabilistic cardinality estimators, such as the HyperLogLog algorithm, use significantly less memory than this, at the cost of obtaining only an approximation of the cardinality.
维基百科上有写的啊:
简单来说就是用正确性换空间。如果你的数据量不大当然用不上。比如你的移动应用有多少个安装(使用 UUID 标识的)。