Yup, when shooting fireworks, it's not uncommon to have long shutter speeds like 8 or 10 seconds.
One reason is that it's often hard to judge exactly when the fireworks will explode, so with a longer shutter speed, you don't have to be exact.
Another reason is that you can sometimes catch more than one blast in a single exposure, making it more interesting.
Thirdly, a longer shutter speed allows you to capture the whole life span of the firework...from the spread to the downward trail of the sparks. Sometimes that looks good, sometimes not.
Of course, there are other factors. For example, if it's not very dark, then a long shutter speed like that may cause you to overexpose the image. Or it might give you a slightly exposed foreground, which might be an improvement over a pure black sky for a background.
Shooting fireworks is one of those things that can be done in very different ways. There really isn't a right or wrong and experimenting can be lots of fun. The only problem is that you don't have all night to experiment because the show is usually over way too soon when you finally figure out what is working for you