I'm sure there are many reasons that were discussed in financial and technical meetings, but here are some possible reasons to consider:
Technology issues: For some years, we had only 2G service available. Devices were built with 2G cellular technology to match. Then companies started shutting down 2G service. Devices were becoming obsolete, and doing so on different schedules in different countries. Then we went to 3G. Then 4G. Now 5G. It would be a bit annoying to keep chasing the latest standard and have devices keep going obsolete, with customers unhappy as a result. The BYOC (Bring Your Own Connection) solution solves this by assuming that the owner will be updating cell phone technology as needed to stay connected.
Cost issues:
a) Not a large part of the total cost of a device, but removing cellular connectivity saves on hardware expense.
b) It's not a free service to TomTom, there's payment to the providers involved, and TomTom have to pass that cost along to the consumer as best they can, increasing the price of their devices when in-dash solutions abound. TomTom doesn't charge for the cellular subscription on a monthly basis, but they do have to pay for it. They must also manage all of the activations and accounts to keep square with the cellular providers. There's a fair bit of overhead involved there.
c) At one time, there were cellular plans that simply didn't provide for any kind of tethering service, or if such service was provided, it was quite expensive to the consumer. At that time, the idea of BYOC would have been a show stopper for a lot of potential customers, and a provided cellular connection got around all of that. Tethering is now no longer a major financial obstacle for most customers.