Yup. I find it useful for subjects that are relatively still, or when you want full control over the focus (e.g. doing controlled focus racks when shooting video), or in situations where AF just doesn't work well (lack of contrast, low light, etc).
It's something you use as an alternative to AF, as Kit mentioned. Comparing auto focus to manual focus, I think that's quite self explanatory. It's described in good detail in the manual. I see it as a guideline to help me MF.
One thing I'd note is that it's something best used with practice as you learn how it works and its limitations. For example, at shallow DOF, it's not a given that FP will always indicate the point that is tack sharp, still gotta use your own judgement and understanding of the lens/camera. With wide angles and deep DOF, it might show a larger range with the peaking, but identifying the actual tack sharp portion can always be a challege, because the DOF is deeper. Bottom line is, expect a fair amount of trial & error, and practice, if you want to master it well.
