Quick answer:
The early taproot of the plant is fragile, and critical to the plants development. So once the taproot starts to grow, you want the plant in soil (or appropriate medium) and you want to mess with it as little as possible.
Yes, once that early root starts to grow you still may be able to get away with taking it out to take a look, but really. . .if you can avoid doing that, you're better off.
Note that lots of people like the paper-towel germination method for this EXACT reason. . .so that they do NOT have to dig through any soil to check on the progress of their seeds! Once the thing starts to grow, they plant it, and are done with it.
If the seed hasn't germinated yet, then it doesn't make any difference if you take it out and look at it, then replant it.