Why does the Catholic church have so many Masses throughout the week?

To answer the question “which Mass is most important,” it’s Sunday. The Catechism states:

The Lord's day

1166 "By a tradition handed down from the apostles which took its origin from the very day of Christ's Resurrection, the Church celebrates the Paschal mystery every seventh day, which day is appropriately called the Lord's Day or Sunday."³⁶ The day of Christ's Resurrection is both the first day of the week, the memorial of the first day of creation, and the "eighth day," on which Christ after his "rest" on the great sabbath inaugurates the "day that the Lord has made," the "day that knows no evening."³⁷ The Lord's Supper is its center, for there the whole community of the faithful encounters the risen Lord who invites them to his banquet:³⁸

The Lord's day, the day of Resurrection, the day of Christians, is our day. It is called the Lord's day because on it the Lord rose victorious to the Father. If pagans call it the "day of the sun," we willingly agree, for today the light of the world is raised, today is revealed the sun of justice with healing in his rays.³⁹

1167 Sunday is the pre-eminent day for the liturgical assembly, when the faithful gather "to listen to the word of God and take part in the Eucharist, thus calling to mind the Passion, Resurrection, and glory of the Lord Jesus, and giving thanks to God who 'has begotten them again, by the resurrection of Jesus Christ from the dead' unto a living hope":⁴⁰

When we ponder, O Christ, the marvels accomplished on this day, the Sunday of your holy resurrection, we say: "Blessed is Sunday, for on it began creation . . . the world's salvation ... the renewal of the human race .... On Sunday heaven and earth rejoiced and the whole universe was filled with light. Blessed is Sunday, for on it were opened the gates of paradise so that Adam and all the exiles might enter it without fear.⁴¹

There are a large number of masses on Sunday (including the Vigil Mass on Saturday evening) in order that the largest number of the faithful can attend: it would not be possible for all the Catholics in a particular church’s “catchment area” to fit into a church if there were only one. I have been told that there is a Catholic church at each corner of Clapham Common in London; and each of those churches has an attendance at mass of over 2000 people per week.

³⁶ Sacrosanctum Concilium 106.
³⁷ Byzantine liturgy.
³⁸ Cf. Jn 21:12; Lk 24:30.
³⁹ St. Jerome, Pasch.: CCL 78, 550.
⁴⁰ SC 106
⁴¹ Fanqith, The Syriac Office of Antioch, vol. VI, first part of Summer, 193 B.

Why does the Catholic church have so many Masses throughout the week?

It is because of what Mass is to the catholic church. Mass is the Single most important commandment given by Christ to do until the end of times (Do this in remembrance of me Lk 22 / 1 Cor 11).

"At the Last Supper, on the night he was betrayed, our Savior instituted the Eucharistic Sacrifice of his Body and Blood. He did this in order to perpetuate the sacrifice of the cross throughout the centuries until he should come again, and so to entrust to his beloved spouse, the Church, a memorial of his death and resurrection: a sacrament of love, a sign of unity, a bond of charity, a paschal banquet in which Christ is consumed, the mind is filled with grace, and a pledge of future glory is given to us" (Sacrosanctum Concilium 47).

The mass is so important to the catholic church, that no-day passes without a Mass said at-least in one place in the world. This is not scientific, but it is folklore that there are three Masses being said every second in the world. (It is jokingly said that many Kings, dictators wanted to destroy the Catholic church. they tried different ways and failed. But all they have to do was to stop them from celebrating Mass.)

For the practicing Catholic, which Mass service is most important?

By the word important, if you mean the importance of the Mass itself, then for Catholics all mass is the same Calvary sacrifice, no one mass is greater than the other.

By the word important, if you mean the importance of participating in the Mass. Then important Mass would be Mass on Sundays and other Days of Obligation. There are specific ranks assigned to certain days. But this does not make the Mass more important (in the sense others are less important) in anyway. But it is important in the sense that they have to attend it.

Where would Bible Study and devotionals fit in?

First part of every Mass always starts with Readings from the bible. This is called the Liturgy of the Word. (Priests are required to read prayers from the bible three times a day. This is called divine office. Along with the Mass, divine office is the only public prayer of the Catholic Church)

On an ordinary day three passages are read. These passages are from:

• Old testament or non gospel new testament. This is made into two separate passages on On Sundays and great feast-days (Solemnities)
• Psalms (Some times, very rarely, other canticles from the bible too)
• Gospel

After this during Liturgy of the Eucharist, all prayers said is based on biblical verses. So, in a way Mass is bible in action.

Do children worship with the parents, or do they have their own little worship area/classroom setting where they learn about God and stuff?

Mass is the primary form of worship in the Catholic church. There is a provision for a children mass where special prayers are said for them But worship is not to be done in a classroom setting. But almost as a norm Children always attend Mass with their parents. It is always recommended to do so. In-fact certain age children have a specific role in Mass called alter servers. Usually in parishes children (not toddlers) perform this role.

There is only one Mass for all. But there are separate Catechisms class (usually after Sunday mass) for Children.