Purification, the creation of sacred space, lighting a fire, and introductory prayers are all good ways to prepare for major prayers. They make the prayers themselves into little rituals, placing them in the context of your entire religious system. They are a very good way to begin scheduled rituals, such as morning or evening prayers.
I can hear you saying, “But what if I just need to pray at a moment's notice?” Don't worry. These preparations are not absolutely necessary. There will be times when you simply won't have the time or the means to prepare for prayer. As an extreme example, when your car is sliding into another on an icy road, there isn't exactly time to do a purification ritual. “Isis, help,” may be all you can get out. And since that is perfectly in tune with what is happening, it is a perfect prayer.
If you have the time and the means, though, something more becomes appropriate. Part of living a Pagan life is doing that which is just right for the moment. Doing a more complete ritual when you can is just right for that moment.
The gods appreciate effort. They like to see that we are willing to take the time to do everything possible. You may find that your prayers yield more blessings if you perform the preparatory rituals whenever you can—not because the gods can't give you what you want without them, but because there is little motivation for them to do so if you can't even make the effort to prepare yourself to contact them. And don't you want to make an effort? Aren't they worth it?
Even if your prayer isn't a request for anything, you will be given benefits—an increased sense of knowing your place in the universe, a stronger awareness of the presence of the gods, a development of your spiritual discipline. When praying, try to make a full-fledged ritual out of your prayer whenever possible. The benefits are real and well worth the trouble.