Aside from "just 'cause" I think the main arguement for a new bike is that it will allow you to do something that you can't do now. I have 7 bikes, each different in its intended purpose. Road, fixed gear, cross, vintage stylin', touring/commuter, hard tail MTB, and dual suspension MTB. And no, I didn't spend a fortune on bikes, most were used and picked up for next to nothing. The point is, is there something you want to do that you can't do now? If you do primarily recreational riding or commuting and the present bike meets those needs then you don't need a new bike. OTOH, if you ride with a club and find you're struggling to keep up and everyone else has a new bike that is 5-lbs lighter than yours, well, maybe you do.

A new bike might be more comfortable if it fits better than your present one, it might be lighter, it might be faster but are any of those things important to you? Almost certainly it will be prettier as new things tend to be. Is it worth the money. Only you can decide.