There can be all sorts of reasons a person may have a very specific routine before a performance of some kind. (Superstition being no small reason for many actors, believe me.)
There are also physical reasons some may have a very precise set of activities before a show. Vocal warm-ups. Stretching exercises. Medicines, and the like.
And of course, some have no routines, (or perhaps rituals is a better word) the precede a stage appearance. I am not one to knock this, but I am here to advocate routine/ritual for the actor. Even if the ritual has no deeper symbolism or practical need.
For those with insomnia, and other sleep disturbances, professionals often recommend establishing a bedtime ritual, which wavers very little from night to night. Brushing one's teeth at the same time, getting a final drink of water, and those similar things. The body and the mind work as one to recognize that sleep is about to take place, and patterns of subconscious "shut down" before sleep begin to take place.
The therapy for a great number of mental disorders has as a vital componant, some sort of routine, to help bring about perspective and clarity.
Religious ritual, and its purposes are obvious.
All of these things apply to the actor about to perform, and I highly recommend either routine or ritual. It will center your focus, activate parts of your mind and subconscious, and in general put you into a place that your whole self knows is "the performance zone". Your performance cannot help but improve in 99% of the cases.
Exceptions exist for every rule, but my observations over the last 9 years of being an actor have taught me that the best performers I have worked with have, at some point in time, engaged in some kind of consistent routine, either in public, (jumping jacks in one case) or in private. I cannot get inside the heads of such people, but my guess is that even if they are not aware of it, the reasons they engage in ritual is that puts their whole being, almost automatically into actor mode. Every facet of them is aware that when these actions are being taken, a performance will follow soon after.
They key of course, is to avoid extremes. Leave room for the unexpected. Do not become a mental slave to a specific song on the MP3 player, or a certain breathing exercise. For routine can turn against you and become an obsession, and obsessions, when not obtained, can actually lead to a very distracted actor.
Still, if you do not have a ritual, get one. Even if it is nothing more than deep breathing 5 minutes before curtain, or always leaving your shoes off until a certain time before the opening of a show. You cannot force superstition o emotional significance into a ritual, but once any given focusing technique starts to yield it's obvious benefits to you in rehearsals and performances, I am willing to bet you won't want to be without it again afterward.