Why science-fiction fleets need a three-dimensional arrival convention

What in-universe reason could explain Star Trek ships arriving in a system on the same planetary plane?

A navigation standard could use the primary star's poles to define an approach plane, but ships should still reorient after arrival because real space is three-dimensional.

The panel separates a television staging choice from a possible in-universe navigation rule. One convention could use the primary star's north-south axis as a common reference, choosing the dominant star in binary or trinary systems. Even then, a vessel should arrive and then orient itself for its mission rather than assume every orbit shares one flat plane. Warhammer 40,000 offers a setting-specific alternative in which warp exits and navigator expectations impose an arrival direction, but the wider lesson is that space cannot be planned as a two-dimensional sea.