Usage of GET - Basic LOGICAL database question

Hi,

I have a few doubts in Logical Databases.

1.) Why should i include a logical database in a abap program.

2.) Can i add more than 1 LDB to a abap program

3.) can Nodes statement used on all nodes on LDB

4.) Please provide a example program based on F1S LDB with small explanation.

Thanks in advance,

Bala.