There are a couple reasons that multiple injections work well for most people. The first reason would be to keep the levels of test in your blood elevated. If you inject test on day 1 (Mon) the level will peak within 3-4 days and start to gradually taper down. If you inject again on day 4 (Thurs) the level rises. The goal is to keep the levels of test in your blood elevated and consistent. Your body will utilize all of the test you put in it (within reason).
The other reason is comfort. Injecting 500 mgs. of test at one time is around 2-2.5 cc. For someone just starting out, this may prove uncomfortable as most people are not used to it, hence the smaller dose done more frequently.
Short acting esters such as prop. do need more frequent injections as most know. Sust. also works best with multiple injections but we have to remember that it is made up of long and short acting esters. If the theory that multiple injections of long acting esters wastes the test., wouldn't you, in theory, be wasting half of your sust.? Something to consider.