What in the purpose of in-liquid-line filters?

When you build a compressor that is supposed to run in an enclosed system you want the coolant medium (and the lubricants) in this system to function optimally during the service life of the system.

First of all, the coolant medium has a tendency to absorb water and this moisture can be aggressive on the internal components of the compressor so you want to remove this once the system is sealed.

Second, you want to filter out any particles that may shorten the life of the system.

Those are the two purposes of the liquid line filter drier in the refrigeration system.