Synastry reads two charts together. Not "are these signs compatible?" That is a shallower question. Synastry reads the specific relationship between two specific charts: where one person's planets fall in the other person's chart, what the planetary aspects between the two charts produce in the dynamic between them.
Not will this work? The question is: what is the shape of the space between these two people, astrologically?
Not whether to stay or go. Not whether this person is the right person. Those are questions astrology does not answer. What it offers: what is likely present in the dynamic between these two people, based on the planetary relationship between their charts.
The most useful synastry readings describe something you already sense but have not been able to name. A Venus-Pluto opposition between two charts matching the quality of intensity and entanglement that characterized a real relationship. Not explaining it. Finding vocabulary for what was already there.
Also useful for understanding relationships that ended or were complicated in ways that did not resolve cleanly. The chart does not assign blame. It describes the astrological conditions: what was amplified, what needs were in tension, what the structural challenges were.
Not a verdict. Information.