It'll be easier to find things. Nothing is more frustrating than having to look and look for a possession you need that is somewhere in a mess of clutter. When you clean the garage out and remove any unnecessary items, you can reorganize in a way that makes you happy. 

There is a decreased chance of pests. Where there is clutter, there are pests. By clearing more open spaces, you will minimize the living areas for any unwanted animals. 

It will be more aesthetically pleasing. After all, who doesn't love having a clean home? 

You'll get peace of mind that the job is finally done. You've probably been telling yourself that you would clean out the garage for a long time, but haven't had the time to get around to it yet. Now's your chance to finally get it done, and feel relieved that you did.