Lets say that there are two cell service providers generally in your area. Provider A and Provider B. Their coverage is not identical, there are areas where there is coverage by Provider A and NOT Provider B, and there are areas where there is coverage by Provider B and NOT Provider A. You have an unlimited account with Provider A, but if your phone connects to Provider B, it costs you $5 per megabyte, $1 per minute, $0.25 per SMS message.
If you set it to automatically choose network and you go into an area covered only by Provider B, then you pay really nasty roaming charges.
If you lock it to Provider A, then you never pay any roaming charges, but you are more likely to lose coverage.