There are several reasons why you might want to reindex all index documents. This includes changes in the Solr configuration how text gets indexed (for example to activate certain features such as stemming) and changes in configuration or code so that different data is sent to Solr. In any case, reindexing a whole index is a very expensive operation and takes some time.

See also the chapter about reindexing in Solr Reference Guide: Reindexing.