[narfforc]

The only person who knew what Bilbo looked like was Gollum. Sauron thought in some way they were of the same kind, and that Gollum would eventually find where other halflings lived. If Gollum had found Bilbo, and murdering him took the Ring, Sauron knew that Gollum was already under the power of the Ring and would not be able to resist putting it on. Once Gollum had the Ring, it would be far easier to detect and catch him