You will want a sim card in Bali! Especially if you are the type who really depends on their phone and google maps. Although you can just download offline google maps on your phone but the issue with this is that it doesn’t always work because of signal and sometimes you really need data. We bought a sim card at the airport when we arrived and it cost 300,000IDR for 15GB of data and 15 minutes of local calling. There was 3 of us so we only bought 1 sim card and we shared hotspot whenever we wanted (careful though.. This drains the battery on the phone who has the sim card). We didn’t get through all 15GB of data for the 9 days that we were there but it was nice to have at our disposable. It was also convenient to have the 15 minutes of local calling because when we would order a ride using the GRAB app, sometimes the driver would call us to try to find us.

P.s. it's probably cheaper to buy a sim card somewhere outside of the airport. Couldn't tell you where though, never saw a location but we also were not actively looking. It's definitely possible because I know many who have! 

To be happy in this world, first you need a cell phone and then you need an airplane. Then you’re truly wireless.
— Ted Turner