What is the need for twist along a wing?
Washout or twist is a method to tailor the local incidence to achieve the ideal lift distribution over span independently of the wing's planform.

For example, if you desire to have a wing with minimum induced drag, you choose an elliptic planform. However, when stalling, this kind of wing will show a nasty roll instability because the lift coefficient is constant over span, causing one of the tips to lose lift first. To improve handling, designers learned to add chord near the tips. This by itself would create more lift at those tips than ideal and increase induced drag. So they added twist such that the local incidence decreases toward the tips. Now the wing has again the desired elliptical distribution, but has some margin at the tips to ensure that stall happens there last.

Note that washout will result in the desired lift distribution only at one angle of attack.  

Washout is also necessary to adjust the lengthwise lift distribution in flying wings. Here the elevons will adjust local lift in order to trim the aircraft, and in order to avoid excessive trim deflections, washout is added so that deflections can be kept small.