When we water our plants in the greenhouse and especially the seedlings, if we just use mains water we can give them a real shock.
The water that comes out of the tap is probably around 5 degrees but the plant and its soil may well be at 20 degrees or more in the greenhouse. By just keeping your watering can or cans full of water in the greenhouse, it will be at the right temperature to avoid shocking your plants.
With seedlings this is especially important, they can easily be killed by a surprise chill.
If you have room for a slim water butt in the greenhouse then you can have a constant supply of water at the right temperature and the water butt will act like a storage radiator, absorbing heat in the day and releasing it at night, helping to keep your greenhouse warm at night.