Volcanologists frequently visit volcanoes, especially active ones, to observe volcanic eruptionss, collect rock and lava samples. Volcanologists study volcanoes in order to better predict eruptions. There is currently no way to accurately predict volcanic eruptions. Predicting eruptions, like predicting earthquakes, can save a lot of lives.
Being a volcanologist is one of the most dangerous scientific professions there is. Many volcanologists have died because of sudden eruptions while they were studying volcanoes.