Hi Thief on the Cross.
I will give you my view on it; others' views may differ.
First, I'm not sure how the area (that you're referring to) became known as "the altar". At one time, it was called things like "the mourner's bench". Charles Finney called it "the anxious bench".
Those latter two names will give you the significance of it. Basically, the idea and purpose for it is that it gives a person (who may be under conviction) the opportunity to respond to that conviction at the earliest possible moment after hearing the word of God preached. The idea being, that if a person possibly waits, they may (or may not) make a decision in response to God's word.
That's a good question. I personally believe that to call that area "the altar" isn't really in keeping with the biblical use of the term. The altar was the place (in the Old Testament) where animals were sacrificed and slain. There was also the altar of incense.
Various people also built altars in the Old Testament prior to the institution of the law. I don't recall if any of those altars were used for anything other than sacrificing animals. They may have been; I just don't recall right now.
In the New Testament (aside from the four gospels), the only altars that are mentioned which aren't for Israel's sacrificing of animals are...
1. An altar to false gods in Acts 17.
2. An altar mentioned in Hebrews 13 that belongs to "us". I think most people would say that this altar is the cross of Christ since that is where he was slain.
3. The altar of God in heaven in Revelation.
Now aside from that, is there a biblical reason for doing this? I would say that's up to the individual church...
My reasons in favor of it:
1. When the people heard the word of the Lord read in Nehemiah 8, they immediately acted upon it.
2. When the keeper of the prison asked Paul and Silas what he must do to be saved, he was told and immediately acted upon it.
Now, it must be understood that in those two examples, there wasn't "an altar" (as we're referring to). They heard the word of God and immediately acted on it. So, people can still act on what they've heard from God's word without an "altar". My main reason for using those examples is that it (the altar call) gives people the opportunity to act immediately after hearing the word of God. However, the actual "acting on God's word" takes place in our daily lives.
My reasons against it...
There is nothing in the bible (that I know of) to describe what we know as today's altar call.
At the same time, there is nothing in God's word describing a Sunday School, Church Choirs, Assistant Pastors, etc. There are many things done today that fall under what I would call "Christian Liberty". There may not be anything expressly supporting them in scripture; however, there isn't anything expressly forbidding them either. So churches are at liberty to use them.
My concern (apart from the name itself) is that "the altar call" has become more than it was intended to be. Some use that time to pressure people. Some use it as a way of measuring a successful sermon. Some use it as a way to compare themselves with other churches. I want to stress "some"...not all!
Now keep in mind, these are my thoughts on it. Others may disagree. In fact, if anyone knows that I'm mistaken in any part of this, I would hope they would point that out to me so that I can make it right.