Blocks always use braces. Expressions use parentheses for grouping sub-expressions. It's the same as many C-like languages. I can't imagine any situation where you would be confused which one you need to use.
Because it's an arbitrary piece of code that needs to be re-evaluated for every input item, like an inline function / lambda in other languages, not an expression that is evaluated once when the commandline is instantiated.
Blocks always use braces, but sometimes you are passing parameters and other times you are passing a block.
If I spent all day as a PowerShell dev I'm sure I would know it, but for something I reach for once or twice a year intuitiveness is a feature I would like to have.
