Answer

I’ve written more than once that in many cases Leibowitz takes a correct principle and goes one step too far with it. The example I usually used was prayer. I completely agree that the motivation is supposed to be the obligation to pray. But from there to the claim that intentions, desires, and the content of the prayer have no value — meaning that one could just as well recite the phone book — that is a very great distance.
I explained that he is indeed correct that even in prayer, commandments require intention, and the intention is to fulfill one’s obligation. That’s it. But above that foundational level there can be further levels, and the more you add, the more value it gives: a desire for connection and standing before the Holy One, blessed be He (according to Rabbi Chaim, this is part of the intention that is indispensable in prayer). The kavanot of the Ari. Directing oneself toward some kind of spiritual results, fulfilling physical desires, and the like. None of these can be the basic motivation for your prayer, but there is no reason to say they are worthless and that there is no point in adding them. It is not for nothing that they instituted this prayer text (praise, thanksgiving, and request), and not the phone book.