Visitors use specific keywords. Their landing page is no more main page, in most case. For example, if I am looking for an Internship job at a web design company, I will use keyword ‘internship web design’. Search engine returns result that direct me to the subpage describing the internship program of a company, instead of their main page.
What is important for a subpage then? Limit number of subpage you have. The less the better. You need to be very sure about the goal or the conversion you want for each subpage. Convert them to your customers.
Subpage should have a brief introduction that tells your customer what you are doing, I bet you have this in your main page too.
Remember, every subpage initiates a process to be completed!