Transformers generate tremendous amounts of heat. This heat can be cooled by air in case of small transformers. In large transformers, more substantial cooling media are needed. Mineral oil is widely as a cooling 
Silicone Transformer Oil has high flash point. Hence, it can be placed closer to buildings as the risk of fire is reduced. Silicone fluid is also self extinguishing. The heat and smoke produced in the event of a fire is minimal. It is not hazardous and environmentally friendly. It can be recycled. Its base is polydimethylsiloxane is a chemically inert material. It does not form sludge and down not break down over its lifecycle. The Transformer has a longer life with reduced maintenance.
The downside is that Silicon Transformer Oils is not biodegradable.
Useful Links
http://www.siliconerecycling.com/silicone-oil-usage-for-transformers/