@Crowned Princess
For some, there may be interwoven reasons, to a greater or lesser extent.
1) Because it feels good to be reminded of faith.
2) Because it feels good to have a ready-made witness tool available always.
3) Because it feels good to identify with other Christians who also use the tattoo parlor.
4) Because it feels good to identify with a Christian family member who already has a faith based tattoo.
5) Because it feels good.
(In the scheme of things, I don't think the idea of "feeling good"
on its own is exclusively a particularly sound reason for doing something. If there is a significant usefulness in witness, however, this might alter the situation a great deal. If this makes sense?)