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It's for optimistic locking. If you attempt to commit some changes to an entity but it turns out someone else has updated that entity in the meantime (as indicated by the version number of your copy no longer matching that in the database), you can abort the transaction or possibly give the user a way of merging their changes with the previous changes. If you've ever used Bugzilla and had a "mid-air collision", this is an example of such a feature.