Everyone who has ever lost a computer file knows how important it is to keep backups. The rest of us should learn from their experience. Keeping current backups ensures that you are prepared for any upgrade of equipment as well as a hardware crash. When the time comes to move your new computer in, all you need to do is copy back to your new hard drive all the files you've saved. Any file, program, document, or piece of information that you need to do your job is important.
Email is important.
Internet URLs are important. Copies of Microsoft Office documents are important. Anything mandated by the UK Model records retention schedule is important. Voyager data files are important. If you haven't looked at an email since it came in, you will most likely not need it.