What's the purpose of the catch block in an ASM_REQUEST_BLOCKING scenario

You can use the more complete form of catch to return the resulting error:

if { [catch {
        do only if  previous was event ASM_REQUEST_BLOCKING
        if { $activeViolation } {
            modify respose header
            HTTP::header remove Content-Length
            HTTP::header insert header_1 value_1
        }
    } err] } {
    log local0. "Error is ${err}"
}

Ref: http://www.tcl.tk/man/tcl8.5/TclCmd/catch.htm