Why is Propanolol (Inderal) given to a patient with Toxic Goiter?

To slow the heart rate. The excess T-4 from the over active thyroid can cause mild to severe tachycardia. It may have some other beneficial effects in hyperthyroidism, can reduce tremor, and anxiety symptoms. This does not treat the cause of hyperthyroidism. It just manages symptoms until definitive treatment is given.

Probably to lower blood pressure