Well....if you had a mixer, then you could monitor you external synths, you could mic up a drum kit (or similar) and then submix to stereo to record via the mbox.....you could patch in reverbs etc via the aux sends.....there is quite alot you can do with a mixer, BUT, the question is this.
Do you need a mixer?
If you are mainly using VSTi's or softsynths/samplers, and you have no outboard synths, then you dont need a mixer to monitor them.
since you have the MBox you can only ever record 2 things simultaneously.....do you need you record more????
The preamps in the MBox are probably better than on the behringer......
Lots questions here, but dont just buy a mixer 'cos everyone else has one', cos the chances are, you may not even need one.
The only thing I use my mixer for is to monitor my external synths, and for it's preamps....but then I have an 8 analogue input soundcard.....so I can actually record all 8 signals onto individual tracks, whereas with the Mbox you can only record in stereo.
Good luck!