The Steed birthsign grants a bonus of 20 to a player's Speed attribute. These bonuses are temporary, once the respective attributes reach 100 this sign has essentially no effect.
(Summary and picture from Official Elder Scrolls Site)
The boosting your speed can be useful if you don't want to bother to have more than one speed skill. However The thief also boosts your speed as well as your luck, and agility.