Aren't we all saints? The Catholic faith teaches that all of the baptized in Christ can be referred to as "saints". 1475 of the Catholic Catechism states:
In the communion of saints, “a perennial link of charity exists between the faithful who have already reached their heavenly home, those who are expiating their sins in purgatory and those who are still pilgrims on earth. Between them there is, too, an abundant exchange of all good things.” In this wonderful exchange, the holiness of one profits others, well beyond the harm that the sin of one could cause others. Thus recourse to the communion of saints lets the contrite sinner be more promptly and efficaciously purified of the punishments for sin (1475).
Also, in 948 we find the following:
The term "communion of saints" therefore has two close linked meanings: communion "in holy things (sanca)" and "among holy persons (sanctii)". Sancta sanctis! (“God’s holy gifts for God’s holy people”) is proclaimed by the celebrant in most Eastern liturgies during the elevation of the holy gifts before the distribution of Communion. The faithful (sancti) are fed by Christ’s holy body and blood (sancta) to grow in communion of the Holy Spirit (koinonia) and to communicate it to the world.
In more narrower terms, Saint Matthew's Gospel refers to the saints who from the dead after Christ's Resurrection as faithful departed who were taken by Christ to heaven. Saint Paul refers to the "saints" who will accompany Christ from heaven when He returns. Saint John in revelations uses the term to the "saints" who are now in heaven praying to God. The Catholic church honors specific, certain Christians with the the title "Saint", which indicates the Church's confidence the individual has died in communion and friendship with God and is now with Him in heaven. We teach that not only do we (humans) not know who is certainly in Hell, we also do not know who is certainly in Heaven. It is important to note, we know the criteria - Jesus - but we are not privy to the information of who is actually where. This is information that God knows. An example would be of your neighborhood who tells you she has accepted Christ and is in communion with Him. You both know the criteria, but can you say with certainty when she passed she went to either Heaven or Hell? No, only she knows, and only God knows.
So how does the Catholic church make the claim we know that certain individuals are in Heaven? Evidence is sought through Canonization. Canonization is a process in which certain evidence is obtained - reliable testimony of the person's holiness, indication that person has drawn others to God, and meticulously investigated and documented miracles occurring after the person's intercession has been asked for.
Why pray for intercession?
We pray for their intercession in our troubles in the same manner in which you ask your fellow family, friends, and Christians to pray for you. The Saints are those who believe to be in Heaven, there with God. Who could be more powerful advocates? The key here is that they are advocates, helpers, sanctified holy creatures who are now fully with God and his heavenly host. As our God is the God of the living, and they are more fully alive than we are (See Matthew 19:29, 25:46, 10:17-22, Mark 10:30, Luke 18:18-30, John 3:15-16).
1 Timothy 2 says
"I urge, then, first of all, that requests, prayers, intercession and thanksgiving be made for everyone. This is good, and pleases God our Savior....who wants all men to be saved and to come to a knowledge of the truth."
Matthew 25:21 says
When a saint enters into the joy of their Master, they are "put in charge of many things"
“His master replied, ‘Well done, good and faithful servant! You have been faithful with a few things; I will put you in charge of many things. Come and share your master’s happiness!’
Ultimately, there is only one person performing the task and only one mediator - Jesus, Christ the Messiah. The Catholic, Eastern, and protestant churches all acknowledge and agree. Only Jesus mediates, only Jesus, God, and the Holy Spirit perform the action.
Why venerate the Saints?
We venerate the Saints for the same reason we venerate great scientists, leaders, founders etc. We give them honor. We admire them, we see the beauty and magnificence in their communion with God and the extraordinary way the lived as Christians.
Romans 13:7 says
“Pay … honor to whom honor is due”
By appreciating the craftsmanship, all accolades go directly to the Craftsman.