Maybe you're still using USB sticks to share files in the office. Or may be you share a Dropbox account that works ok for the first 10 or 20 files, but when you try to share large, important files (such as your QuickBooks company file) you realize that it takes longer and longer.
With your own private server you can share files, backup your computers, store scanned documents, photos and even protect your business from ransomware (that nasty malware that encrypts your files and asks a ransom for them).
You can even access your files from your mobile phone, from your laptop or tablet. This can be your own private cloud.