In the preparation of 1-bromobutane from butanol why is the yield not 100%.

Also, why is sodium metabisulfate added in the isolation part.

It is quite hard to control a reversible reaction, and your question is no exception.

Have a look at this; especially the 'notes' part.

Preparation of 1-Bromobutane (http://chemistry.gravitywaves.com/CHE301/Preparation%20of%201-bromobutane.htm)