Why: You must see reliability problems to fix them. The simple log-log plots make the models visible. The task of the reliability engineer is to put favorable cusps on the Crow-AMSAA trend lines to make failures come more slowly and thus decrease the long term cost of ownership. If you're doing your improvement job correctly, you'll never have many failures until you have a cusp.
When: The plots are useful for development tasks (where they first were used) or to long term operations. They work for safety programs, plant improvement programs, environmental programs, or for cost problems. Use the plots as show me, don't tell me, how the projects are proceeding and the key metric in the form of line slope is easy to understand and easy to communicate in less than 60 seconds.
Where: They are used for technical development issues or for management reviews. A picture is worth a 1000 words for getting management's attention for focusing on a problem. Likewise the charts are highly useful for showing the reductions in failures that have occurred from making a desirable and permanent fix.
These definitions are written by H. Paul Barringer and are also posted on his web site at www.barringer1.com