It is presumably an incentive for a Texas resident student to graduate from a Texas public university in the minimum possible number of credits for his/her degree (up to 3 extra credits allowed), to allow the state subsidy of the public universities to be stretched to cover as many students as possible (instead of a smaller number of students taking more credits worth of courses each).
http://www.collegeforalltexans.com/apps/financialaid/tofa2.cfm?ID=447