You do, but only if you created your own computer files. This does not necessarily hold true if you have a designer or print vendor create them. The print industry trade customs state that files are tools of the trade.
This is no different than if you contract a carpenter to build a wooden porch for you. Let’s assume you have agreed to have the porch built for a certain amount of money. What happens when the job is competed and you find out that the carpenter has extra nails and boards? Are you going to ask him for the left over items or allow him to put them in his truck? Was there something in writing on the estimate stating that you would recover any unused materials?
This is the same principle with trusting your print or graphics vendor who created your digital files. It must be stated clearly on the written estimate as to who owns those files especially with complex projects. You may someday want to use those files again in some other fashion. You could ask for a copy of the file but if your print vendor gives you an Adobe PDF it may be useless if you have to make future edits. It gets worse if you get unprintable Adobe PDF files. An unprintable Adobe PDF file is a digital proof that can be seen on the computer screen but will print as a hardcopy original.
It can become very interesting when you wish to change designers or print vendors. Don’t be shocked if your vendor argues with you on releasing the files or charges you for locating and transferring your duplicate files. There are some sharks in our industry like any other profession, so buyer beware.