What should I know about the sym file, and why back it up?

In a partitioned database, the sym file is a string intern pool which grows every time a new distinct symbol is introduced and persisted to that database. The sym file cannot be regenerated (unless you know the exact order in which your data was loaded from the inception of your database) and does not automatically shrink when symbols are removed from the database.

See this solution to compacting a bloated sym file.