You have to have some sense of history here.

Early iterations of FD cable routing used a housing loop to an arm on the FD, similar to how RDs still are today. The Rd cable was routed through a section of housing over the BB, or a through a small clipped-on noodle.

Later the housing loop and noodle was replaced with an open clip mounted at base of the downtube passing the wires over the BB.

At some point a few of us started looping the FD cable under the BB shell, and up between the chain stays. Some also ran RD cables this way, but some couldn't because the rear loop stop was brazed above the chainstay.

Later a few builders inverted the chainstay stop, so both wires could be routed under the BB. With more people routing this way, some started refining the concept with guide loops like the OPs tandem. Also as the trend started moving beyond the fringe, people started worrying about the paint, so either better noodles, or curved C-channel guides were brazed on, while those without real provisions would simply trap a piece of housing or tubing down there.

When it moved to the mainstream, owners were more finicky and wouldn't accept the cruder methods of the past, plus builders got tired of the extra work involved in cutting, curving and brazing noodles, so eventually the clip was introduced, making for a finished look, with less work fro the builder.

The reality is that the guide isn't necessary, as demonstrated by all those who routed below the BB before the guides existed.