Are you talking about the add-on Nokia GPS vs. 3rd parties?

If you are talking about the 810 vs. a dedicated GPS, the Nokia tablets are general purpose computers, in a sense. In many/most cases, there will be dedicated devices that will do the job better and cheaper.

People will use the nokia because:
1. They don't want to carry/have multiple devices. In a car situation, convergence really helps when there is limited space and/or power outlets.

2. Theoretically, having one device can make things easier. Want to map out a contact, go to your contact list, press <map> and next thing you know, your route is planned. Of course, this completely depends upon the software.

3. One device can be cheaper in many instances, if you are willing to live with the compromises. Will your GPS allow you walk into any WIFI hotspot in the world and truly access the internet? You may need 2 devices then. Obviously, GPS + PDA + Internet Access can easily cost more than an 810.

4. Open Source and the Nokia Development community: Most devices are closed, and the only software you can get on it is what the manufacturer gives you. With the tablets, the possibilities are endless as to software development, and you can even roll up your own sleeves if you want to. I'm really not sure how good GPS manufacturers are at updating their software (and at what cost), but I'm gonna guess they are going to side on either trying to sell you a new unit, or charging for updates.