I'm getting cavities filled, getting an iv sedation. is this really necessary?

Yes and no: Is what really necessary? Getting cavities filled? Yes. Getting IV sedation? No, unless you want to have it done that way. Most patients do well with just local anesthesia. If you need something more from a mental\apprehension\fear standpoint (and not from a dental standpoint), you can consider the use of "laughing gas", oral sedation, etc. Discuss options with your own dentist. Get a 2nd opinion.

Usually not: Cavities can often be filled without any anesthesia or sedation. In most cases, local anesthesia is sufficient.

No: Only under management concerns would you have an IV sedation. For filling cavities a local anesthetic is all that is normally required.