Its an EU-Ops requirement to add 50ft to the MDA.
As mentioned above, you cannot decend below the MDA so when you start your go-around the aircraft will still decend around 30-40 feet before it will start to climb away...this 50 feet is a buffer.
With the DA(H) its the latest altitude you can start a go-around or I should say make the decision to go-around.
Make sure you read the plates as some NPA now use DA(H) instead of MDA.