Once the initial index has been rebuilt, Joomla normally keeps the index updated when content is created, edited, published/unpublished, or deleted. For this to work, make sure the Content – Smart Search plugin remains enabled; Joomla specifically notes that content changes will not update the Smart Search index if this plugin is disabled.
For SP Simple Portfolio, the same principle applies as long as its Finder/Smart Search integration is enabled and correctly responding to Joomla's content events.
You would normally only need to run Components → Smart Search → Index manually when you have, for example, migrated/upgraded the site, installed or updated the search integration, cleared the Smart Search index, or noticed that existing Portfolio items are missing. Joomla's documentation treats running the Indexer as the step for building/rebuilding that index.
So, since your search is working correctly now, you can leave it alone. As a quick test, create a temporary SP Portfolio item with a unique title, save/publish it, and search for it. If it appears without manually indexing, automatic updating is working correctly.