You're always turning blood cells over and producing new ones.
In a survival / evolution sense, there's no benefit to letting go of something that keeps you alive.
But the effects are fairly short lived and include a decreased ability to transport oxygen around the body (so decreased cardiovascular performance and decreased recovery within muscle tissue), decreased blood volume - which can lead to a difficulty in maintaining blood pressure under extreme circumstances, like doing a high rep workout.
You'll mostly have negated the effects within 2 - 3 days, and completely within 7 - 10 days. I wouldn't run a marathon within a week of giving blood.