Why the lifespan of each relationship must be recorded and kept？

[shuijian-xu]

the lifespan of each relationship must be recorded and kept so that the results from queries will faithfully reflect history. An example of this in the Wine Club is the relationship between customer and sales area. If a customer moves from one sales area to another, it is important that the previous relationships of that customer with sales areas are retained.

[shuijian-xu]

Retrospection = true for relationships