1 Thessalonians 5:25 Brethren, pray for us.
One has to ask: why would apostles of Jesus Christ ask fellow Christians to pray for them, instead of simply asking Christ directly? Unless one answers this question and accepts the answer, one will never understand Catholic teaching on the "communion of the saints" (Apostle's Creed). It's not superfluous, so what is its purpose? The same principle is at play when the apostles ask their fellow Christians to pray for them, as when we ask ours.
One answer, certainly, if not the answer, is that the apostles, like any Christian, have a certain duty before God - the duty to live as Christ wants, with the graces given them by Him (cf. the Parable of the Talents). Given this, it follows that if a Christian wishes to offer their own suffering, prayer, sacrifice for another, that God will honor that by granting their fellow with the graces that would be afford them themselves (what else does "This kind cannot by expelled except by prayer and fasting" Mk 9:29 mean, or, "I rejoice in my sufferings for you, and I fill up in my flesh what is wanting with regard to the sufferings of Christ, for the sake of his body, the church?" Col 1:24). We are one body, and so can help each other:
1 Corinthians 12: 12-26 For as the body is one, and hath many members; and all the members of the body, whereas they are many, yet are one body, so also is Christ. 13 For in one Spirit were we all baptized into one body, whether Jews or Gentiles, whether bond or free; and in one Spirit we have all been made to drink. 14 For the body also is not one member, but many. 15 If the foot should say, because I am not the hand, I am not of the body; is it therefore not of the body? 16 And if the ear should say, because I am not the eye, I am not of the body; is it therefore not of the body? 17 If the whole body were the eye, where would be the hearing? If the whole were hearing, where would be the smelling? 18 But now God hath set the members every one of them in the body as it hath pleased him. 19 And if they all were one member, where would be the body? 20 But now there are many members indeed, yet one body. 21 And the eye cannot say to the hand: I need not thy help; nor again the head to the feet: I have no need of you. 22 Yea, much more those that seem to be the more feeble members of the body, are more necessary. 23 And such as we think to be the less honourable members of the body, about these we put more abundant honour; and those that are our uncomely parts, have more abundant comeliness. 24 But our comely parts have no need: but God hath tempered the body together, giving to that which wanted the more abundant honour, 25 That there might be no schism in the body; but the members might be mutually careful one for another. 26 And if one member suffer any thing, all the members suffer with it; or if one member glory, all the members rejoice with it.
The church is an 'organism,' not a set of individual, independant 'cells.' Therefore, so ask for the help of fellow Christians - even if they are of the angelic, not human, nature - is to admit and acknowledge and implement this fact in your life as a Christian.
It happens also to be the practice of the church immemorial.
There is also the aspect of our being more worthy of the audience of angels and saints than God Almighty, by default, and at all times, without exception, making recourse to angels and saints in heaven, the more humble option, when beseeching God's mercy or grace.