I highly doubt that it's 40 gigs of apps - but I'm pretty sure it is local backup snapshot. This was just discussed in another forum. You can either not worry about it - because the system will free up space from local snapshot as needed, or you can turn it off in terminal.
As far as cleaning up your old unused apps - just like old paper files you have at home, you should at some point dump what you're not using. Lots of people get sloppy about files on their system and doing spring cleaning once a year can help drastically - both with your system performance and your life organization.