Because: 1) I need one in a car about 3 times a year. The phone does a good job and costs nothing extra.
2) I use my phone on my bike and 3) on foot much more. I'm not carrying an extra costly device around. In fact, a lot of times 4) I don't know I'm going to need it beforehand. The phone is always ready.
Recently I was a passenger in a car where the driver had to first fanny about dialling in letters with a knob. Starting with the name of the city we were in. We had to sit in the underground car park while he did it. 5) With my phone, the route is already sorted in comfort, and checked with email or papers that I've got the right place before we get near a car.
6) Recently it even helped me find where I parked the hire car in a foreign city. 7) And I didn't have to pay rip-off sat nav rental costs for some old tatty sat nav, that I'd have to lug around whilst sight seeing.
The 'slightly' smaller screen is clearly not 'too small'. Let alone 'far too small'. And for me, dedicated satnavs have at least 7 show stoppers, it's just not an option.
Does that help you understand?