Better Discovery Scanners mean you can get 'unknown' hits to add stellar bodies to your nav list, so you can select them and move towards them more easily and also target them for scanning.
The ranges to locate stellar bodies when you 'fire' the discovery scanner are:
Basic - 500ls, Improved - 1,000ls, Advanced - All stellar bodies in the star system
When you target an 'unexplored' stellar body, and you are close enough (variable, depending on the mass of the target) you will get the swirling icon that means 'scanning' and after some seconds it will populate with a name and a type instead of 'unknown / unknown / unknown'.
If you have a detailed surface scanner on your ship, this second scanning process will give even more information, viewable in the system map view. This will also make the payout for that body more than it would have been without the detailed surface scanner.
TL;DR: better discovery scanners - easier to find things, detailed surface scanner - more cash for the things you find