There is an old saying in chemical labs: never heat a closed system. If you are heating up a glassware system or a metal container system to perform a chemical reaction at high temperatures and the system is sealed up gas-tight to the atmosphere, then as the chemicals and gasses inside the system are heated up they will expand. If there is no place for this excess volume to fill other than the existing rigid system, there will be pressure put against the system walls. In the case of glassware, most of it is not designed to operate under pressure so you run the risk of glass shards exploding which is obviously a dangerous situation. If you are heating up a metal system that is designed to operate under pressure, it is usually designed to safely operate under a certain pressure limit. If the pressure exceeds this limit for any reason, then there can be a dangerous explosion. A safety valve allows for a controlled emergency release of pressure before an explosion can occur. It is an important safety feature of closed, pressurized chemical systems.
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