Any random point (a location) in space can be defined by a coordinate in any number of coordinate systems. The stargate system uses its own coordinate system based on 39 constellations (for a Milky Way stargate), symbolised by chevrons on the stargate. But whether these chevrons symbolize constellations, or even if there exist dimensions in that specific coordinate system is not relevant to answer your question. Fact is that there are 39 points in space known by the stargate.
Accept that a stargate can only establish a wormhole, or travel path, if it knows an origin and a destination.
Now, the simplest definition of such a path would be just 2 points: the origin and the destination. But how many stargates are there? And how many symbols are on a stargate again? Clearly, two points to define the path isn't going to work.
Somehow the destination point has to be constructed out of the 39 points known to the stargate. For instance, take two points which form the ends of a 'line' and the stargate calculates its middle which would translate to the destination point.
The question then rises: is that enough precision? A combination of 2 out of 39 results in only 741 possibilities. So 2 points are too few to give enough resolution.
One step further: take 3 points to define the destination point. The stargate system would calculate the triangle center from those 3 points to get the destination. Well, 3 out of 39 leaves us with 9,139 possibilities: again not enough.
There is no evidence of the Wilky Way having more then 9,139 stargates (or even more then 741), but note that the destination point does nót point to a stargate, but to a random point in space (a stargate could be anywhere). The stargate system just picks the stargate closest to that destination point. (But that is explained in the series later, not in the original movie.) Clearly, space has more then 9.139 locations.
4 points result in 82,251 possible locations, 5 points in 575,757, and finally 6 points result in 3,262,623 possibilities.
And (apparently, but also kind of obviously) a Milky Way devided in at least 3 million sections guarantees a possible unique location of a nearby stargate. So, only with six defining points there are enough posibilities to define enough destination points.