The placement of shutoff valves at critical points throughout a system will permit the easy field  replacement major system components.  One example is the use of ball valves on either side of the filter-dryer.  Hand valves are also used to isolate different parts of a system when using multiple compressors, evaporators, etc. when only one is to be operational at a time. 

RParts hand and ball valves are a "packless" design.  This means they require no stem packing and will not leak even after years of constant service.