Most layers (lyr) would reference only one feature class. They include the selections and symbology for that feature class. I know of two good uses.
Some add-on software that creates GIS data can produce layer packages for exporting and sharing data. This enables receivers of the data to get only what they need. A similar situation is when a complex map package doesn't quite work at the receiver's end; the problem layers can be re-sent as layer packages. Layer packages are faster and more reliable to produce than map packages because they are smaller and simpler. We occasionally have had to send layer packages to printing firms for this reason.
A great use is to save a layer's symbology before its source feature is redesigned, renamed, or moved. Layers with broken links lose a lot of information such as classified symbology. If you have a lyr package created before the conversion, you can extract it, open it, and use what you see there to re-create any map layers that did not get updated with the new source.