If the variable being evaluated contains spaces, there can be problems.
For these cases, double quotes around text that may contain spaces will force the surrounded text to be evaluated as a single token. The double quotes will be counted as part of that lexical token.
As an example:
exten => s,6,GotoIf($[ "${CALLERID(name)}" : "Privacy Manager" ]?callerid-liar,s,1:s,7)
The variable CALLERID(name) could evaluate to "DELOREAN MOTORS" (with a space) but the above will evaluate to:
"DELOREAN MOTORS" : "Privacy Manager"
and will evaluate to 0.
The above without double quotes would have evaluated to:
DELOREAN MOTORS : Privacy Manager
and will result in syntax errors, because token DELOREAN is immediately followed by token MOTORS and the expression parser will not know how to evaluate this expression, because it does not match its grammar.