Defining SQL Command

I am trying to learn more about the Defining SQL Command in the Database Expert.

I understand from what I've read that it can be used to create a virtual table that can provide access to a smaller number of fields and records than the entire database, so that accessing the data is quicker. Also, if you need this data structure across reports, you can save the command to the Repository.

I understand that such a virtual table would be helpful with a large database having a lot of fields. It would reduce the number of fields in Field Explorer, which would make designing a report easier.

I don't understand why it would be necessary to parcel out certain records, though. Doesn't the Record Selection Formula do the same thing -- it only pulls into the report those records that are needed? If someone could explain this further, I'd appreciate it.

Are there other benefits (or disadvantages) of the SQL Command functionality that I'm missing?

If there are any readings out there that discuss the SQL Command in Crystal, I'd appreciate the reference.

Thanks.

Gary