The short answer is that we are Christians, and we like the Bible. However, just in case you are looking for a little more of the backstory…
The teachings of Scripture give our worship a basic liturgical pattern that has been fleshed out by the experience and traditions of Christian congregations for two thousand years. This pattern helps to give The United Methodist Church its sense of identity, and links us to the universal Church.
The practice of including more than one Scripture reading in the worship service is one that differentiates many liturgical denominations (United Methodists, Episcopalians, Presbyterians, Lutherans, Roman Catholics, etc) from many of the evangelical denominations (Southern Baptists, Pentecostals, Non-denominational Churches, etc) that often choose to only focus on one reading of the Scriptures during a worship service.
There are many people who do not encounter the Scriptures outside of a Sunday morning worship service. Consequently, United Methodists and other mainline denominations believe that it is important that each service make use of the ancient and ecumenical practice of including four readings, as embodied in the Revised Common Lectionary. This helps to ensure that over a period of time, people hear representative readings from the totality of Scripture.
The ancient and ecumenical order of these readings as embodied in the Revised Common Lectionary is as follows. The sequence of readings is normally ordered so that the sermon is preceded by the primary text to be preached.
First Reading (usually Old Testament)
Second Reading (from the New Testament, but not from the Gospels)
Hymn or Song
Gospel Reading (a reading from Matthew, Mark, Luke, or John)
Because in the reading of the four Gospels we are addressed by the words of Christ and experience this as an encounter with the living Christ, many Christians prefer to stand during the reading of the Gospels as an act of respect for the Christ who is addressing us.