I have blogged a few times about the importance of routines to my son, and he is not alone with this, a lot of autistic people find routines help them to deal with day to day life, they struggle when things are unpredictable, when things change.
So having a good routine helps them to be able to predict what is going to happen and takes away a lot of the stress that day to day life can cause.
It can in itself cause problems though, because as much as the predictability can help them remain stress free, when something new needs to be added into the the day, it can cause major anxiety.
Introducing new things can be very difficult and it is advised to give plenty of notice, plenty of planning for a change can help them accept the change when it happens and help to reduce any anxiety, talking about it before hand can be very helpful.
As can introducing things such as a now/next board to help them move from one task to another.