Martin (1992: 196-7):
But with conditionals, the Effect may or may not be desirable; if and unless do not code it as one or the other:
[4:66] If you go that way,you'll get there by six/you'll get lost.
[4:67] Unless you go that you [way]you'll get there by six/you'll get lost.
Purposives on the other hand always code the Effect as desired or feared. In [4:68] Ben will be read as wanting to get lost, and as not wanting to get there by six in [4:69].
[4:68] Ben went that wayso that he'd get lost.
[4:69] Ben went that wayfor fear of getting there by six.
In other words, fear and desire are implicit in [4:66] and [4:67], but can be read in where the context indicates they are needed to make sense of the conditional meanings made explicit. Fear and desire on the other hand are grammaticalised in [4:68] and [4:69].
 Clauses related by the logico-semantic relations of condition and purpose do not construe a cause-effect relation. Condition means if P then Q, purpose means because intention Q so action P. It is reason/result — because P so result Q — that construes a cause-effect relation.
 Because the logical meaning of purpose involves intention, not desire and fear, desire and fear are not grammaticalised in clauses logically related by cause: purpose.
 Because the logical meaning of condition does not involve desire and fear— cf if it has three sides, it is a triangle — desire and fear are irrelevant to the discussion of clauses logically related by condition.