The following is from a piece published last year —

As the name would suggest, couples therapy is intended for couples. But sometimes, only one partner is willing to go. “The resistant partner may fear being vulnerable, think that therapy won’t be helpful or be uncomfortable talking about their problems.” “Regardless of the reasons, I’d say it’s still helpful if the other partner comes in for support.” 

My take. This is so true. Often one party may come initially but then over time the other would also joins in.  this is often with good results for the couple’s relationship.