Many android phone have the low internal memory problem. Because all apps are install in phone memory. The problem can be solved if the apps can be installed directly on SD card making a 2nd partition.
Also apps perform better when they are on internal memory rather than on SD card because there is an easier access to their resources.. but not all phones have enough internal memory to move all apps to internal memory .
so keeping apps on SD and creating a link to them ensures better and faster performance of all apps even in devices with low internal memory.