To make sure the extension only indexes products that are supposed to be indexed, all products are processed when performing a reindex without the indexing queue enabled.
If a product being processed is disabled, out of stock, or invisible, Algolia performs a delete operation even if the product is not present in the Algolia index at all.
To avoid these extra delete operations, you should enable the queue. With the queue enabled, only products that meet the requirements are indexed and pushed to Algolia. The indexing queue makes use of atomic reindexing.