Do you mean for private interest? If so, I would agree that prison labor should only be used for public benefit. And this labor should be part of the sentence.
Somebody who had worked for a recognizable tech company is far more hireable than somebody who is Self Employed or who has worked for the government.
Here, we're talking about preparing someone for the job market when they leave. Hence, these are two separate concerns. You cannot substitute the former with the latter.
To me, it looks like a net benefit for the public, the department of corrections, and the inmate.
If you are worried about the inmate being allowed to build up savings that they can use when they are released, then that's on the judge. If the inmate has met their restitution obligations, then I don't have a problem with them being allowed to leave prison with savings that will enable them to get back on their feet again.