I know that many churches "do" baptism differently than we do. I understand that some baptize infants because they believe the act imparts grace to the child and helps on the road to salvation. Some baptize infants because they view it as a sign of the covenant between God and His people, something like the OT circumcision. I can see the merits of both of these arguments, and I'm not here to say those ideas are wrong; I just want to say why we do things the way we do.
First, why baptize at all? This is actually the harder question for me. It's just one of those things that churches do. So let's dig a little... The first time we hear about anyone baptizing was John as "Mark 1:4 John did baptize in the wilderness, and preach the baptism of repentance for the remission of sins." I'll be honest: I don't understand how getting wet remits ones sins. But I'll accept that baptism is a symbol of the spiritual work done in a person's life.
Second, why "believer's baptism"? We believe that baptism is symbolic of repentance and faith in Jesus, and only a person of certain age and mental capacity can experience those. Therefore it is fitting to baptize people who have professed faith in Jesus and repentance of sins, as "Acts 2:41 Then they that gladly received his word were baptized: and the same day there were added unto them about three thousand souls." And "Acts 8:36, See, here is water; what doth hinder me to be baptized? 37 And Philip said, If thou believest with all thine heart, thou mayest."
Third, why immersion? It's my understanding that the word baptism actually implies immersion. But beyond that,
"Romans 6:3 Know ye not, that so many of us as were baptized into Jesus Christ were baptized into his death? 4 Therefore we are buried with him by baptism into death: that like as Christ was raised up from the dead by the glory of the Father, even so we also should walk in newness of life." Immersion best pictures the death and resurrection, at least to me.
I think that about covers it. Thanks for sitting in on my lesson. :-)