The main reason for me to use Grab while on holiday in Thailand was the fact that not getting scammed by a taxi driver in Thailand was almost impossible for me. I'm not sure if this is an issue for everyone, but I dare to say that as much as 50% of all taxi drivers in Bangkok just drove away the moment we asked for 'on the meter'. And this is saying something because we have used taxis at least twice a day for 2 weeks.

The second reason was that not 1 of the non-Grab taxi drivers uses anything like Google Maps. So I was never sure if we actually ended up at our hotel or somewhere else. Also because our hotel (Aetas) had (at least) 2 locations in Bangkok it made it all worse. This is assuming that a taxi driver actually knows where to go, we've also encountered multiple times the situation where a taxi driver didn't know where to go.

Grab on the other hand always provided the amount upfront and every driver had Google Maps for navigation.

Edit: I'd rather use the official taxis, but the number of conveniences Grab added with a single app was worth it to me.