Hermetically Sealed Transformers are transformers which are airtight i.e. they do not allow the air from the atmosphere to enter the transformer. These transformers are totally filled with oil. Preventing the entry 
of air protects the oil from oxidation and consequent deterioration.
Since the transformer is totally sealed, water cannot enter the transformer. This protects the insulation from moisture.
The transformer has low maintenance as there is no need for testing the oil or the insulation for a period of 10 years. This reduces the life cycle cost of the Transformer.
Since, there is no ingress of water or air into the transformer, the aging of the transformer insulation and oil are considerably reduced.
Hermetically sealed Transformers are used applications where it is difficult to conduct maintenance such as Offshore Platforms. Hermetically sealed transformer are more expensive to purchase. However, since they have lower maintenance costs and downtime, the life cycle cost of these transformer is cost effective as compared to conventional transformers.