Domestic flights in Nepal are notoriously unsafe. Indeed, the safety problem arises from the altitude, with airstrips located in the mountains, but also from the recurrent weather changes in Nepal.
Nevertheless, it remains a very practical means of transport, given the long car and bus journeys. Indeed, even though Nepal is a small country, journeys can be quite long, as the roads are virtually nothing but curves in the mountains.
Flying is therefore a great way to avoid spending too much time on the road and make the most of your trip. It's also a great way to get to the starting points of treks quickly.
Although domestic flights are notoriously dangerous, accidents are rare.