FE lock can be used in any metering mode. I use evaluative mode (canon) most of the time, especially when I am not using FE Lock. So I would say that the FE Lock is more useful for spot, partial and center weighted.
I think it very important to understand what these metering modes are and try them out in different situations. That is when you will know when to use FE lock or not.
Your camera manual can tell you what is possible. Obviously, in Manual Exposure mode it's redundant or not working since you are setting the exposure manually.
When to use? Obviously when the exposure metering must be separated from the focusing. Decide which part of the frame needs correct exposure. Select the necessary metering mode, use FE lock to keep the exposure settings, recompose, focus, shoot. Check your manual for the details (also for how long the metering result is kept and when it will be overwritten / replaced).