The name of the lectionary tells us a couple things- it's been revised, and it's shared in common between a lot of Christian denominations. The earlier Common Lectionary had an Old Testament reading, a Psalm, and a Gospel reading for each Sunday, the RCL has added a New Testament reading as well. This Lectionary is used by a lot of denominations, including the Episcopalians, the Methodists, the Presbyterians, and the Reformed churches. The Catholic church uses a slightly different one, so that very often they will have the same readings as the RCL, but occasionally will have a one that's different.
If you've ever mentioned to a friend, "Oh, my pastor preached on this story last week" and your friend said, "Hey, so did mine!" it's very likely thanks to the RCL, or another lectionary.
So, why the lectionary? Well, it does a lot of things.
- It saves time. Most pastors I know take about one full day out of their work week to write their sermon as it is. Choosing texts on the fly would make that a lot harder. Also it helps a pastor keep track of what sermons they've preached where, so they can keep some variety going.
- It makes sure readings fit that Sunday. The Bible is a very large book, as you've likely noticed, and choosing three related readings and a Psalm that also flow with the church year, what was read last week, what will be read next week, and everything else is a complicated project.
- It keeps pastors from fixating on one topic for too long. There's an old legend I heard once, which probably isn't true, about a newlywed pastor who preached for three straight months (about a hundred years ago) on what made a good wife. I can't even imagine being that wife (or, for that matter, that pastor!). Pastors are as likely as anyone else to get a "bee in their bonnet" and this keeps them from obsessing too long on one thing in the pulpit.
- It does a pretty good job of cycling through the Bible, and so helps educate the congregation about the Bible. Like I said, it's a three year cycle- each year focuses the Gospel readings on either Matthew, Mark, or Luke, and readings from John are spread through all three years. The other readings are also varied.
- Professional discussion. Many pastors take part in weekly Bible studies, where they look at the readings for that week and talk about what they plan to preach on. For many pastors I know, this is a necessary part of their professional growth, a great networking opportunity, and also a deeply appreciated social outlet.