An example: Suppose you had a script that used WMI remotely to gather/update information on a number of remote servers. Instead of doing the required operations on one server at a time, running each invocation of the script block in parallel. The ability to run script blocks in parallel has been a huge reason many folks use Workflows (which are not part of PowerShell 7). Foreach -Parallel does away with the need to use workflows just to get parallelism.
Consider the following snippet: