Some search programs rebuild on a timer — nightly, weekly. Fine for cold archives. Annoying when you just saved a file and it doesn't appear yet.
Real-time updating watches the filesystem and patches the index as files change. On a busy machine that's trust versus frustration. Save, Alt+Tab to search, open. If the new file isn't there, you'll uninstall within a week.
How watchers work
Most hook into NTFS change journals or ReadDirectoryChangesW on selected roots. CPU cost on SSDs is negligible. USB drives are trickier — some tools pause on removable media to avoid constant rescans.
Full rescans still happen as safety nets: corrupted index, restored backup, cloned disk. Manual rebuild button saved me twice last year.
When scheduled scans still make sense
Read-only archive drives you mount once a month don't need live watchers. A manual rescan on plug-in is enough. Know which volumes are hot and which are cold, then match the update strategy. See also when not to index a folder.
See also: Cloud sync folders and local search — what gets indexed, SSD, NTFS, and why search feels instant now.