Profit is a difference between the amount earned and the amount spent. In consulting business, earnings are usually revenue from projects while spendings are the cost of delivering these projects: salaries of consultants and overhead (non-project-related expenditures).
Without profits, consulting companies could not invest money in marketing, sales, training, services engineering, and knowledge management. Consulting businesses without profits are more like self-employment or nonprofit organizations. A typical profit margin for a consulting firm is in the range of 15%-25%.