It's an idiomatic Tcl thing. E.g. `expr` (the standard word used to evaluate infix expressions like `1+2`) does the same exact thing, handling the expansion itself and expecting the caller to use {} to ensure that a variable expansion not incorrectly treated as a bunch of operators. Similarly, when you're writing the condition for a loop, you need to use {} to delay expansion so that the loop word can do it anew for each iteration. One can argue that this is somewhat error prone, but at least there's a consistent pattern here, and once you know what it is and why it exists, it's pretty straightforward.
