You don't really need a sys.txt. If iGO runs well, don't worry.
Sys.txt can be useful when your device needs (device specific !!!) like to let a hardware button do his job (zoom in/out or rotate screen, ...) or set GPS port/baud, or ....
Also, when you want some specific settings, some things can be set in sys.txt, examples : the size of the triangle cursor, the warning distance for speedcams, etc. Some of these needs are answered by using skins.