These people should automatically be deported. If this happened I think less would try to come here illegally. By holding them in detention centers it is using up more U.S. funds than what it would to deport them. In the long run I think we would come out ahead by deporting them.
Every day costs taxpayers more money. Detention centers are just a stalling tactic and ties up US citizen workers involved in this process, when their time could be better spent deporting those who shouldn't have been here in the first place. It would also send the message that we intend to uphold our own immigration laws. Stalling on the matter just sends the message that the US is wishy-washy.
Some illegal immigrants don't have the necessary paper work that is required to deport them so the authorities require time to prepare all that necessary paperwork and in the meanwhile those illegal immigrants have to be kept somewhere because they can't be put into the prison or if they're let loose for the time being they might hide around.
Detainees in these facilities may actually be asylum seekers or otherwise legally able to stay in this country. These facilities, while crowded, are necessary to ensure that compassionate justice is served for all, which is one of the main precepts of the U.S. Immediate deportation short-circuits the principle of due process.
There are various reasons to why illegal immigrants arrive in the USA. There are cases where these immigrants have at times, gone through a lot of the processes and cleared them and yet they are proclaimed illegal. Immediate deportation will lead to no investigation in cases where there has been an error and as such, it's best if detention facilities remain.