We all come into our relationships with an amalgamation of sexual preferences, experiences, learning, urges, desires, fantasizes, hangups, and patterns. None of these things spontaneously arrived in you fully formed and fleshed out as you are today. Sexual development creeps along from the moment you are born to the end of life with numerous points of exciting, and sometimes difficult, changes. No two people will have the same sexual path in life and even our own experiences may be enough to challenge our sense of competence, confidence, and enjoyment.
Sex therapy is about helping individuals, couples (or other relationship units) find solutions to difficult blocks in the path of their sexual development. Despite sexual difficulties being common experiences throughout the population, many people still feel quite alone, confused, broken, shamed, and much worse when they experience these blocks.
The absence of a specific identifiable sexual problem is not the equivalent of creating a great sexual relationship between you and your partner(s). Neither is it true that having a sexual problem strictly leads to a poor sexual relationship.