For which of the following reasons would David Laugherty, owner of the Laugherty Associates law firm, want to know the total costs of a job (serving a particular client) ?

I don't know about the complete options available, but I know the purpose of finding the total cost of the job, which is to charge the clients the right fee depending upon their time taken and cost of each job. That's the purpose

of knowing the cost of the assignments. Another purpose includes to maximise the profits by opting to jobs which has higher contribution per unit. By meeting the demands of jobs which has higher number of contribution per unit, first, makes the firm more profitable.