While it has clearly been shown by the virility of the virus so far that travel restrictions will not stop the inevitable community spread of the illness, and that their effectiveness as impeding the spread of the virus is small, the aim of the policy is no longer to stop the virus, but to delay it. This was confirmed as current policy in the UK's response to the virus by the Chief Medical Officer, Professor Chris Whitty, on March 5th.
There are a few reasons why this is the goal of many governments. Firstly, if the peak of the pandemic can be delayed, this allows plans to be put in place for management of the crisis. For example, in the UK, plans have been floated that would allow final year medical students and trainee doctors to take on hospital duties during the nine week period that the virus is currently predicted to have the highest effect on the NHS, in order to mitigate the strain on the service.
Secondly, delaying the spread of the virus allows time for a vaccine to be developed. While research on this is progressing quickly, it will still take time for the vaccine to pass clinical trials and be approved for human usage. In addition, as @Patricia Shanahan points out in the comments, the effectiveness of existing drugs already approved for treating similar respiratory diseases can be evaluated, which could be ready sooner than developing brand new drugs or a vaccine.
Finally, as health services are acknowledged as being under the most strain during the winter months, the best time for the pandemic to peak would be in the middle of summer.
To this end, the policy of travel bans, while having been shown to be relatively ineffective at preventing the spread of the disease outright, is being used to delay the spread of the virus as much as possible, to allow national governments to prepare for the full impact of the peak of the predicted pandemic.