The day ticket lets you use unlimited bus and tran services for one day within the valid area. Are you travelling with a four-legged friend? You can also buy a day ticket that includes your dog!
The day ticket has the advantage that you can pass it on to someone else, provided it is still valid after you have finished using it!
From tariff level 2, the fare for the day ticket is equivalent to the fare for two single tickets. So from tariff level 2 it is better to buy a day ticket than two singles - even if you only want to go there and back.
From tariff level 8, it is acually cheaper to buy a day ticket.