If you use SharePoint on Office 365 or an on-premises server, you've probably had times when you've wanted to access your files from your Windows desktop. You can do this with the OneDrive for Business sync client, even though it's not a feature of the regular modern OneDrive app.
Microsoft's main OneDrive app that comes preinstalled with Windows 10 can sync files from personal, work and school accounts. This includes OneDrive for Business files and SharePoint sites that are part of Office 365 Business subscriptions. However, the new app can't yet sync SharePoint document libraries, an important and highly popular SharePoint feature that lets you store, organise and share files.
To sync a document library, you'll first need to open it in the SharePoint app online. With the OneDrive for Business app installed and open on your PC, click the "Sync" button in the SharePoint toolbar. In the prompt that appears, click the "Sync now" button and approve the prompt to open OneDrive for Business.
When OneDrive for Business launches, you'll be shown the name of the library and SharePoint site that you're trying to sync. Choose a folder to save the library to on your PC by clicking the "Change" link. Once you're setup, press "Sync Now" to start downloading the files in the library. You may be required to sign in with your Office 365 or SharePoint credentials if you've never synced from the site before.
OneDrive for Business will keep you informed on progress as it begins to sync. Once it's finished, you'll be able to access your files from File Explorer, as if you're using OneDrive with a regular personal or business account. If you need to remove the library in the future, right click the OneDrive for Business icon in the system tray and click "Stop syncing a folder." You can also pause syncing or add a new library directly from a URL.
There are a few limitations around document library syncing that you should be familiar with. Unlike the modern OneDrive client, you can't selectively sync folders within a library. If you have a lot of files but only want to access a few offline, you could try sorting them into two different "offline" and "online" libraries.
Additionally, the OneDrive for Business client will only sync up to 5,000 items in a single library and 20,000 items in total. Combined with the 2GB file size cap, these restrictions can be problematic if you have a lot of data stored online. There isn't a simple way of working around the issues so you'll need to plan how you store files to make sure you don't hit them.
Syncing a SharePoint document library to your PC takes a bit of effort but allows you to access all your files natively without using a web browser. Document library sync is the only remaining unique capability of the last-generation OneDrive for Business client app. It's expected that Microsoft will eventually fold the feature into the modern OneDrive app, simplifying the procedure.
In the meantime, you'll need to keep both sync clients installed if you want to download document libraries. Microsoft's support pages provide a complete rundown of which OneDrive version to use for most common use cases.