Company should focus on existing customers primarily and potential customers secondarily who have not taken products from organisation.Is it true?

yes and no, depending on the industry in which the company participate. Some times new clients means new discontinued relation in cases that customers will buy once (consumers products) but in other cases where customers will continue to have a relation with the company it is better to maintain a focus with them. 

in B2B where it depends on the volume of sales, if my current customers are making enough profit and i am working on marginal profit.

or if I am trying to increase my profit.