Well - if you need to ask...then you do not understand Forge - or realize the limitations of audio within Vegas.
Mind you - if you are not doing any hardcore editing...Vegas fits the bill fine....I know a lot of guys who do not but audio in Vegas and it works for them....but for some of us who are in the deeper end of the audio pool....Vegas is not equipped to handle half of what I do...
Batch processing, spectral editing, bit depth conversion...I could go on and on....
Waveform editing (to individual sample level). ie 'surgery' to a level you can't do in Vegas. Stand-alone conversion of files to different formats or specifications. Workflow based on audio waveform editing, rather than 'event' editing.
And CDA, for which Vegas (or SF10 for that matter) are really no substitute.
Seems like batch editing and CD architect are the most popular reasons, neither of which apply to me anymore (I used to use CD Architect all the time!). As for the rest, sounds like if you have a good source, Vegas does all that is necessary.
I don't use Sound Forge anymore, since Sony has gone a different route than fits my needs, but I do use Adobe Audition extensively throughout my workday. There are plenty of reasons for having a good destructive editor as a companion to Vegas.
Mostly it's surgical -- remove a pop there, replace a syllable there, correct spatial bugs -- but also for "pure" sound design; start with white noise and make a shotgun blast.
If you don't feel the need for it, you either have perfect source material or it's not in your job description to work with audio to that depth. Nothing odd about that, everyone is different.