A lot of smartphone makers get around to releasing kernel sources whenever they feel like. It might be a month or two after a device is released or updated, but it happens. OnePlus, to its credit, is much faster. The OnePlus 5 has only just been announced, but the open source files are already available for download.
The kernel source won't be directly useful to users of the phone, but it's a big help for ROM developers. That's a small community for most phones, but the OnePlus 5 is very mod friendly. OnePlus' phones probably have a more active modding community than any other brand right now, so there are plenty of devs interested in these files. The kernel source can also be of use for developers troubleshooting apps on a particular device.
The download contains all the code necessary to get to work porting existing ROMs to the OnePlus 5 and to make new ROMs for the device. The download is available right now on the OnePlus GitHub page.