Why is IV drip used for sinus infections?

In severe cases. Iv antibiotics are used in cases where the oral medication is either not available or not effective. If the infection is cultured and the organism is sensitive only to IV medications then that is the only therapy.
Rarely. In times when there is a patient who is immunocompromised (weak from chemo or from a disease) or has significant lung problems like severe asthma or cystic fibrosis. Iv antibiotics are used for sinusitis. However, this is not common in someone who is otherwise healthy. In cases where a healthy person has difficult to treat sinusitis, a culture taken from the sinuses can point to the right oral abx.
It's generally not. There are only a few instances where IV antibiotics are indicated to treat a sinus infection. Occasionally a bacteria will be cultured from the sinuses which is sensitive to antibiotics which can only be given via IV. In most other instances, other options than IV meds exist.
Sinus infection. Not sure about what you are asking, but here is my attempt. Iv (intravenous) drip is not really used for sinus infection. Normal saline which is the most common IV solution is used to wash out the sinuses as squirts by yourself or in sinus surgery to clean out!