Whenever your product changes, for example you modify it or even remove it magento is notifying about this event all extensions.
Blast Search Lucene is trying to keep its index up-to-date so if event is fired - index gets updated.
Magento indexers (magento 1.4.x and above) can run in two modes (AUTO and MANUAL)
- MANUAL UPDATE - means that you are responsible for running indexer from time to time, magento just notifies you that something has changed and indexer must be run
- UPDATE ON SAVE - magento updates indexers for You - so nothing to worry about