I was kind of viewing "help desk" and field service platform as the same thing. We use our platform for both. But then you bring up a good point in that when we were looking for a platform, there were some "help desk" systems that seemed to worked great for internal use or for customer phone support, but they were not very conducive for providing field services. Things like allowing for products and costs to be added to service tickets and custom fields for warranty and non-warranty customer's, for example.
There seems to be two distinctions for a services platform in the technology world:
Help Desk and Professional Services Automation (PSA) software.
In a nut shell it seems PSA does what help desk does, but has added features and functions for the service providers that also have field service personnel.
By the way, I found this website very helpful when looking for a PSA platform and this is where I found our current platform that we use.