Judy, the reason you need actual Bear Spray is because it shoots the stream a much farther distance than regular pepper spray. You don't want to to let them get too close and if they seem like they are not going to back off, you want to spray them before they get too close to you.
Lindsay can probably give you another reason why bear spray is preferable to pepper spray. Bear spray is expensive $40-$50 per can. We only take it when we are going hiking in Alaska, and have never had to use it, so we are pretty novice when it comes to bear spray. We did encounter a bear on a trail in Colorado a few years ago. They tell you not to run, but I ran back down that tail as fast as can be!
(we didn't have any bear spray with us)