Search index is a utility to help re-building the search index that drives most of product data retrieval on storefront.
It is not meant to be used often as this may disrupt customer experience whilst it is rebuilding the index. This is especially true for shops with large catalogs as the process can take some time.
The platform automatically triggers reindexing on a productsGlobalIndexProcessorJob schedule, which is recommended to be set once a day. It runs early in the day to ensure that the indexes are freshly rebuilt for early morning visitors. If you have automatic daily imports then schedule should be set to trigger after all daily imports have taken place to ensure that index contains the latest data.
It is sometimes desirable to refresh the index manually, say if some manual changes have been applied to make last moment changes. In most cases this is not necessary as individual product data, prices or inventory updates would immediately trigger a single product reindex. If however a manual import was used then it may be desirable to trigger full reindex and this would bypass auto reindexing.