Couples therapy gets recommended reflexively for almost any relationship trouble, but it's not a universal fix, and it's not right for every situation. It tends to work best when both people genuinely want the relationship to continue and are willing to be honest, including about their own contributions to the problem — not just showing up to prove a point or to have a professional referee an argument.
A relationship is generally a reasonable candidate for therapy when the core affection and respect are still intact but communication, habits, or a specific recurring conflict keep getting in the way. Fundamentally incompatible life goals — wanting kids versus not, differing views on monogamy, geographic non-negotiables — are harder for therapy to resolve, because those aren't communication problems; they're genuine mismatches that no amount of better listening will reconcile.
Therapy is also poorly suited to relationships involving ongoing abuse or coercive control. In those cases, joint sessions can be actively unsafe, since the abusive partner may use the sessions to further manipulate the narrative, and individual support (not couples work) is the safer path.
A useful gut check: if you're hoping therapy will fix the other person, that's usually a sign the relationship itself may not be salvageable. If you're hoping it will help you both understand each other better, that's a much stronger foundation to work from.