Answer

This consideration is not relevant to most other commandments. Specifically with regard to a sukkah, we sit in a sukkah in the summer for shade even without any commandment. But nobody just puts on tefillin for no reason, not for the sake of a commandment. This is somewhat similar to the law of one who is barred by a vow from benefiting from a spring, compared to one who immerses in a spring during the summer for his own enjoyment. One of the halakhic implications of this distinction concerns the rule of “an unspecified act is presumed to be for its required intent,” meaning circumstantial evidence that makes explicit intention unnecessary: if someone sits in a sukkah in the summer and does not do so with the intention of fulfilling the commandment, then according to the view that commandments require intention, he has not fulfilled the commandment. But with tefillin, even if you do not consciously intend it, you have fulfilled the commandment, because it is obvious that this was done for the sake of the commandment.