In Chapter 19.3 (ForEach), it's implied that you will need to use Foreach-Object instead of a ForEach loop if you want to pipe your output from the loop to another cmdlet. The output from a ForEach loop can easily be made to accomodate the pipeline simply by executing the loop as a scripblock:

.{ForEach ($x in $y){
Do something with $x
}
} | Get-StuffFromPipeline

That is a handy tip, although it certainly adds a level of complexity and isn't something readily apparent to a beginner.

It's admittedly not something you see done very often (normally you see the result set to a variable, or more fequently an empty array initialized and then results added to that as they are produced within the loop). I didn't quite grasp the way script blocks accumulate objects and then return them to the pipeline until I got to looking into them further. Once I figured that out, other constructs began to make more sense, and using them became more intuitive.

I understand this may be somewhat outside of the intended scope of the book......