If your water pressure is too high it will put unwanted pressure on flexible hoses, mixer cartidges and inlet valves. It may also cause water hammer. Too low and you wont want to be in the shower when a second tap is opened. A new pressure limiting valve will solve both of these problems in most cases.
Appliances and water heaters may not be running efficiently due to incorrect commisioning techniques. There may be a gas leak. If you smell gas shut the supply off and call to have the problem investigated.