[tjiayu (OP)]

Many people will think of the negative effects of when mentioned of volcanic eruptions. But what are really the benefits that these eruptions can bring about?

[SeanB]

Fertile volcanic soil would be one.

[CliffordK]

I saw a program that suggested that the active nature of the crust of the earth including earthquakes and upwellings of soil was responsible for bringing metals and ores to the surface of the planet.  Without this we might not have ever had the copper/bronze/iron ages.  Volcanoes tend to mix the minerals so perhaps not as important of an ore source as metamorphic rock and mineral veins.  However, perhaps one can't have a tectonically active planet with mineral veins without also having volcanoes. 

Volcanic Glass would also have been important in the development of early civilization and trade.

Hawaii would be 17,000 feet underwater without volcanoes. 

An irregular landscape including mountains and valleys my be important for controlling weather patterns including wind and rain on the continents.  The mountains may also be important for storing water, and distributing water throughout the year, rather in pulses.  However, this may not differentiate by the origin of the mountains.

[Mazurka]

There is evidence that the Romans used pumice as a lightweight aggregate when constructing large domes.

They also used volcanic ash as a pozzolan in their lime mortar.  A pozzolan increases the strength (and particularly in the case of non hydraulic lime mortars) and reduces the curing time.

[Andrew K Fletcher]

As Clifford points out the emergence of new islands and an increase in fertility of soils as the soft rocks become weathered with ash to produce decent soils. Islands are also reshaped by eruptions as lava spews out and flows into the ocean extending the land area. The canary Islands, once very fertile land was covered in ash and farming ending at a stroke,is slowly beginning to recover some soil fertility and crops are now grown on the slopes, and forest is growing on the tops of volcanoes. Produce includes some interesting vineyards and impressive wines.

A volcanic eruption often removes all traces of plant animal and human life or at least evicts them for a relatively short time. Ascension Island is a great example of what can be achieved when working with nature on a volcanic Island http://www.bbc.co.uk/news/science-environment-11137903