One reason which some theists, especially Christians, will offer for thinking that the existence of their god is important is that belief in a god is good for, or even necessary for, social order and moral behavior. For hundreds of years, Christian apologists have argued that without a belief in a god, basic social structures would disintegrate and people would no longer find reason to act morally.
It is a shame that so many Christians (and other theists) continue to employ this argument this because it's so bad. The first point which should be made is that it obviously isn't true that their god is required for good social order and moral behavior - most of the cultures in the world have gotten by just fine without their god.
Next is the question of whether or not belief in any god or higher power is required for morality and social stability.
There are any number of objections which can be made here, but I will try and cover a few of basic ones. The most obvious thing to point out is that this is nothing but an assertion, and empirical evidence is clearly against it.
An examination of history makes it evident that believers in gods can be very violent, especially when it comes to other groups of believers who follow different gods. Atheists have also been violent - but they have also led very good and moral lives. Thus, there is no apparent correlation between belief in gods and being a good person.