If you bought a laptop from dell, the default battery for it is usually just a 6-cell battery. A 6-cell will only last for around 2-4 hours depending on use, but a 9-cell can last up to 4-6 hours. Aside from that, there are several advantages that a 9-cell battery has over a 6-cell.
Advantages of a 9-cell battery:
1. More usage time
It can store more power which will result in more usage time. This comes in handy especially if you're using your computer for travel.
Since it's a bigger battery, the laptop will be raised a few degrees, giving space underneath the laptop for more airflow and a cooler system
3. It's Ergonomic
Raising the backside of the laptop a few degrees, people who type on their portables won't have to worry about that flat awkward position anymore.
There are other advantages but then getting a 9-cell is better for the usage of the system than a 6-cell. The only disadvantage I could think of is that it's a bit heavier and it might not look good on the laptop since it's bulkier. But choosing over use-ability and good looks, I'd go for a better performance system than a good-looking one.