Most brides use florals in their centerpieces as well as the bouquets for the girls and bouts for the guys. Hiring a florist can be a huge chunk of your budget! Flowers are expensive.
We hired a florist, but that was because I wanted flowers that I wouldn't be able to buy myself. Plus, I didn't want to deal with the stress of putting together flowers the morning of our wedding.
Flowers are something that are cheaper to DIY than it is to hire a florist. The downside though is that you need to put them together the morning of your wedding or the night before. That can be a very time consuming task. You would definitely need to do a trial beforehand to make sure you can do them nice enough.
You could also use fake florals at your wedding and you could make everything whenever you want before the wedding. Fake florals can be just as expensive as real flowers though, so I don't recommend doing this if you're thinking about doing it for a budget standpoint.
Final thoughts: A florist is a want!