I've always wondered why we should ask others to pray for us if God hears the prayers even of one--and especially if he knows our needs even before we ask of him. Having others pray for us might seem to be redundant, or if we were to take it to the extreme, even faithless.
One Scripture that comes to mind regarding the prayers of many is from Revelation, where the multitude if prayer is likened to incense being poured out to God. There is no indication that these prayers somehow grab his attention better or that they might get lost otherwise, but rather that just as incense is a way of worship, so also the prayers of many combine to form collective praise to our God.
Asking people to pray for us may then be less about maximizing the chances that God might answer our prayer and instead more about creating an opportunity where more people might worship God. And how does that happen through prayer? By asking others to enjoin in our requests, we give opportunity for more people to see how God answers our requests. By crowdsourcing our prayer, we collectively reap the rewards. Our concerns become others' concerns, and likewise our answered prayers become others' answered prayers.