In my experience, when I was “rebaptized”
it had to do with "making a profession of your faith, making your faith public, and not being ashamed to confess with your mouth Jesus as Lord. It offered no graces and was only symbolic, it had no sacramental significance.
Like I said, when each church can be Baptist (including flakes like the Westboro Baptist church), you get a lot of different views on things with no central authority to declare anything. That’s why they’re still arguing and re-arguing everything. The Baptists around here, Chicago, that I’m familiar with are MB, and NBCA. These do claim that baptism washes away their sins (hence the familiar tunes “There is a Fountain” and “Are You Washed in the Blood of the Lamb”), although some may say it’s the decision to seek baptism (to believe) and that the baptism itself isn’t as important (kind of like the Catholic view of baptism of desire, the important thing is the desire and if circumstances prevent, then the desire is paramount). But the baptisms I’ve witnessed amongst these folks are rich in symbolism with the baptizee wearing a white robe and the reverend also there usually in the font as well. Even more moving is the ceremony I saw on TV once where they were using a river for baptisms (as would surely be more accurate from a historical viewpoint if one wants to imitate John the Baptist and his baptism of their savior.
I can only speak from my own experience but the Baptist church I was baptised into insisted that it’s members be baptised in living water, they once avoided using a baptistry for a fellow in a wheelchair by taking him to a lake.