Not sure why but has been done for at least since the 1920s.

That's not what was imagining. If that is what OP had in mind- ignore my comment. I love a nice frieze like that- it's a very nice finishing touch. I was thinking of the vertical stripes people apply willy-nilly. Yup, OP had a horizontal stripe in mind. I see she says across. I will delete my snarky comment.

Rita I did not think your answer was snarky at all, I was just saying odd things sometimes have a long history. I did not pick the above design, someone did 90 years ago. I have no opinion one way or the other. ;) I thought your comment was truthful. I also think people use an accent tile that they might love but can not afford to use enough for a whole shower....

Historically, the accent (and I guess the answer is: accent for interest or definition), was mostly incorporated near the top or the bottom of the wall, often separating field tile from finishing pieces, like moldings or bullnose. The accent piece (and it was often just a narrow liner accent or single row of mosaic) was generally used in other parts of the bath as well, including on the floor. Sometimes it was on the bath wall and continued into the shower for continuity. A few examples:

In the past 20 years or so, the horizontal accent became a "thing", often a strip of mosaics, frequently about eye height. We don't see many clients looking for that anymore. Mostly, if they incorporate this at all, they now want a larger "feature" panel, that often extends down or across the wall, but most often vertically:

But many showers are minimalistic or just classically "plain". Personal preference. I still think a very finely detailed shower, with carefully installed tile and carefully selected and "planned" finished edges and transitions, has more visual and long lasting appeal than "decoration".