It makes the file smaller and more efficient to run queries against.
SQLite's VACUUM behavior basically just recreates the entire database from scratch in a temp file, then replaces the live file with that. With databases, you issue a vacuum command to defragment a database (reclaim space from deleted/updated rows, re-sort the data, etc). The file is only about ~100MB (on my computer). Spotify stores a local SQLite database file (by default on a Mac it's at ~/Library/Application Support/Spotify/PersistentCache/mercury.db ) It likely just stops Spotify from VACUUM'ing the SQLite database it has.
