Something is useless if it fails to produce whatever product we use it to produce.
For instance, if the head breaks off of a hammer, the hammer can no longer be used to produce what we use it to produce—namely, the product of nails-being-compressed-into-wood—and it has thus become useless. We could attempt to fix the hammer to make it useful again: perhaps some super glue will help? We could also find a new way to make the hammer useful: perhaps the handle could be used to keep a window propped open? However, if we find no use for the hammer, if it is completely useless to us, we will surely dispose of it.
Take another example: a man has for years now been feeling very dry whenever he prays to God. In short, his relationship with God is no longer filling him with the very pleasurable feelings of love that filled him when he first converted to the faith. Since his relationship has failed to produce the product that he wants, the relationship has thus become useless. He could attempt to “fix” the relationship to get it to produce good feelings of love again: perhaps more candles would help stir his emotions? He could also find a new use for his relationship: perhaps being a member of the parish could boost his social status in the community? However, if he finds no use for the relationship, if it is completely useless to him, should he surely dispose of it?
No, of course not! Although loving God can be useful in many ways, the core of the relationship is useless. One does not love God in order to produce or obtain something else; one does not love God as a means to some further end. Rather, the end goal of loving God is simply to love God. When we reach this love, there is no need to reach out for anything else. We have already arrived at the All, whose love is sufficient in and of itself. We need nothing else, for God alone suffices. We can rest in his peace.
But I have calmed and quieted my soul,
like a child quieted at its mother’s breast;
like a child that is quieted is my soul.
O Israel, hope in the Lord
from this time forth and for evermore (Ps 131:2-3).