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A spacetime diagram represents events using spatial and time coordinates, often with horizontal and vertical . A particle's world line records successive events on its trajectory; lines describing the propagation of light define the light cone.
The axis convention matters when reading a slope. With vertical, a constant-velocity trajectory has , so the drawn vertical-to-horizontal slope is . Light rays then lie at when the two axes use equal length scales. A stationary object has a vertical world line.
Such a diagram makes special relativity geometric: changing an inertial frame changes how events are assigned spatial positions and times, while causal relations remain constrained by light propagation. A diagram showing only one spatial coordinate suppresses other motion rather than proving it absent.
Answer: it can give some qualitative intuition on what is larger/smaller happens before/after based only on arguably more intuitive geometric considerations, without requiring you to do any calculations, see e.g. Figure "Spacetime diagram illustrating how faster-than-light travel implies time travel".