As taken from WIKIanswers.com
Logical file is a type of file that provides a view of the data stored into the Physical file. It does not contain data. A logical file contains only an index to the data stored into the physical file. we can also update the physical file thru logical file
The only reason you would need another member in a logical is if you physical file currently has multiple members. There is a 1 to 1 ratio of accsess path information vs. member.
Unless of course you use a single logical member to access multiple physical members.
A logical file is a view over physical data. It can present an alternate index, a subset of the fields in a physical file, reformatted data from the physical file, a view of the data from multiple files, &c.
The logical file member points to one or more physical file members (I can’t remember off-hand what the limit of physical file members per logical file member is).