Why should I collect the bonus too?

There is a new gameplay element int Team Fortress 2 - Gun Mettle Update, so called contracts. I know that I can get 2 of them per week and I can collect weapons after completing the contract. But anyway, if you fulfilled the contract, in the end there is an optional "Bonus" which is harder to achieve than the normal contract.

I don't have any idea what this bonus looks like. I have achieved my contracts and I just traded them in after I had the bonus too. But it took a bit more time than the normal contract. Does anybody know whats the difference between a normal contract and a bonus contract? Is there a higher chance for better items or anything like this?