Volcanic eruptions put find dust particles into the atmosphere. These are needed for water to condense around. Water condenses around these particles and, when heavy enough, falls as rain/snow. Without volcanoes, we wouldn’t have as much rain.
Volcanoes help form new land and produce very fertile soil. Ever see pictures of tropical islands? That lush greenery is due to decayed lava. Those islands were also created by volcanoes. Hawaii is still being “created” by erupting volcanoes.
Volcanoes help to cool the atmosphere. The sulfur gas combines with water in the atmosphere, creating microscopic droplets that can stay in the atmosphere for years. The effect of those aerosol droplets is cooling the lowest level of the atmosphere, which is the level in which we live and breathe.
Volcanoes also produce raw materials such as sulfur, copper and gold. Volcanic material is also ground down to make cement.
Volcanoes produce water. Did you know that there is water inside rocks? That’s right, every rock you use has water inside it. No, it’s not sloshing around, and no, you can’t stick a straw inside and suck the water out. Rather it’s locked in as hydrogen and oxygen (H2O). When rock is melted, this water is released (stream). That volcanic cloud you use when a volcano erupts is mostly steam. Some is from the land, but some is from the magma underground.
Hawaii is basically one huge volcano, so if had not volcanoes... There would be no Hawaii.
Subduction... I have not seen it used to mean retraction or withdrawal - whoever gave it that meaning ignored its etymology so warped it, almost reversing it! Sub = below. Duct roughly = to carry, as an aqueduct is a water-carrier; or indeed the simple word "duct" as it is, for a "pipe".
Induct is a sort of opposite, in the sense of entry, as in being inducted into the hall of fame. It is also a form of Induce - hence cause or bring about. An electrical transformer works by the current in one coil of wire inducing a current to flow in the other.
Subduction though, is a geological term. It is process in which one plate of the Earth's Crust is forced below another, down into the molten but extremely viscous Mantle below. Usually, it is an oceanic plate sinking below a continental plate.
There, it partially melts, and gigantic globules of the molten rock mixed with water-vapour and gases from the organic sea-floor ooze dragged down with it, slowly force their way up through the overlying Crust. Where some of this magma breaks surface as volcanic lava, it is very sticky thanks to its chemistry, and the gases effervesce like a shaken fizzy-drink bottle. So it creates extremely violent, explosive eruptions or terrifying 'pyroclastic flows' of intensely hot, toxic gas and ash that rush down the volcano's slopes and destroy anything they engulf. Think of Pompeii....
Before that stage though, there is so much friction between the two rock masses that they move with a sporadic, stick-slip action; each slip causing shock-waves we know on the land surface as earthquakes. On which, another abuse of a word: an earthquake has a focus and epicentre, and epicentre does not mean centre despite ignorant politicians thinking so!
The Pacific Ocean's floor plates are being subducted below the true edges of the continents - not the coasts we see but the continental shelves - on both sides of the ocean, and this is responsible for the "Pacific Ring of Fire" of violent volcanic eruptions and earthquakes. The Mediterranean Sea is also closing, hence the similar events along parts of its Northern coast.
Hawaii though is a very different beast. I think it's actually the world's largest volcano but I may be wrong there. However, I do know that it is in the centre of a Crust plate, so a bit of an anomaly. The likely explanation, which I think is still debated by professional geologists, is that it is above a "hot spot" in the Mantle, possibly the top of a rising convection-column in that molten but very sticky rock. The Earth's internal heat source is the nuclear decay of Uranium.
The volcanic lava from Mantle rock is fairly low in gas and water, and is also relatively runny, so it comes out in comparatively benign eruptions characterised by fountains and long rivers of molten basalt-type rock.
Shuhak 