You gain the general use of a computer directly including a mouse, a larger screen, a hardwired network connection, etc. You also gain the power of Logger Pro, including video tools, gel analysis, and other features of Logger Pro or Logger Lite.
However, many people prefer to use LabQuests without computers because LabQuests do not need much of the maintenance headaches such as virus protection, regularly scheduled IT updates, the possibility of students accessing inappropriate material, etc. Also, don't forget that LabQuests are splash resistant and shock resistant. If your goal is to have a device whose primary purpose is data collection and analysis, LabQuests are generally more cost effective than purchasing an entire classroom set of computers.