If you leave them as-is for internal linking purposes, that internal link won't be of much use if it's not crawled by Googlebot often, or visited by users.
I would go through all these pages getting little to no traffic, add them to a spreadsheet, and determine if they are worth keeping or not on a page-by-page basis. It will take some time and effort, but I would hesitate to just blanket remove a bunch of pages without first checking if any of them could be valuable in some way.
Check if there is similar content on the site you can consolidate and set up redirects for. If there is no similar content on the site and the content could be relevant/useful to potential users, optimize it and try to get it to perform better (then it really...