Graduate education is expensive.
Few universities have the resources to provide all graduate students with full tuition and living stipends. Most, like Boston University, must find ways in which to provide their graduate students with funding to cover their tuition and expenses while they pursue their academic programs. In the case of research assistants, particularly in the sciences, federal grant regulations allow stipends paid to graduate students assigned to the laboratory's funded project to be charged to the grant. These two devices fund a substantial amount of graduate education. If universities did not utilize these devices to support their graduate students, opportunities for graduate education would be greatly diminished.