Laptops are nice for light computing tasks such as word processing, but there are a lot of applications that just aren't going to run well on a cheap laptop. There's also the issue of hardware failure, malware disasters, etc. Students are not, in general, competent sysadmins. The computer lab is critically important when your laptop's hard drive crashes two days before the due date of an important project. On a university supported filesystem, your data is backed up and the failure of one PC is not a problem because you can just use another one. Also, university site licenses for software can be cheaper than buying individual licenses (even with an student discount).