The goal is to have the deprotonated oxygen attack the halide in nucleophilic substitution, kicking off the halogen, and attaching itself to a new carbon chain.  
If one uses halides that are sterically hindered with accessible protons in the beta position, the alkoxide in this reaction with act as a base, especially if it's bulky, and an elimination side product will result.  If this happens, the hydroxyl group on the alkoxide will just reform and a useless separate alkene will form.  The goal is to have the deprotonated oxygen attack the halide in nucleophilic substitution, kicking off the halogen, and attaching itself to a new carbon chain.  FTP, name this reaction that prepares molecules of formula R-O-R, known as ethers.
If one uses halides that are sterically hindered with accessible protons in the beta position, the alkoxide in this reaction with act as a base, especially if it's bulky, and an elimination side product will result.  
Williamson ether synthesis