The English term service implies graciousness, helpfulness and to a degree selflessness. To serve is to be humble. Serve stems from the Latin word servitium, which meant the condition of a slave. Service, at its roots involves one person serving another or several. It is inherently personal. The term service in the context of American business involves the notion of servitium, to respond to the needs of your customer, to serve that customer personally and individually.
But service also anticipates compensation: payment, customer loyalty, growth of the business. Service is both personal and commercial. They go hand-in-hand. Impersonal service seldom leads to commercial success. Personal service without fair compensation is servitude. And, indeed, some business relationships are so one-sided that the one serving feels more like a slave than a free person.
To consult means to seek advice, to refer to, to take into account, to consider, as one would consult an attorney or a physician. To consult also means to exchange views, to confer. As with service the term consult has its roots in Latin: consultare, meaning to deliberate, counsel, consult or take counsel. To consult means to advise, to recommend, to suggest, to provide an opinion about what could or should be done in a certain situation or in response to a certain problem.
The consultant, therefore, is the expert applying their knowledge and expertise to improve the situation of a customer. But, essential to consulting a client is understanding their needs, their situation. This is done by first consulting with, meaning listening to that customer.