Specifically, the structure of H1 visas limit the prospects of highly-skilled employees to a subset of all possible domestic employers, since not all employers in the US have the financial means and/or legal expertise to sponsor visas.
If, rather than forcing recently-graduated PhD students (to take one example) to consider only companies that sponsor visas, we could allow them to apply and receive their I-140s with a high probability and at a low cost. Under this system:
- they can take more time to search for jobs and prepare for interviews
- they can afford to negotiate more aggressively with offers, since they won't be expelled from the country if they reject the first offer that comes their way
- they can switch companies, or start their own companies, with no external restrictions beyond the obvious financial ones
In general, their prospects as employees would be greatly expanded, and their base of negotiation with employers on the whole would improve.
Hope this makes sense.
Edit: The above reasoning may not apply to employees from countries with reciprocity agreements with the US who do not wish to give up their permanent residency in their home countries (if those countries' reciprocity agreements would require that). I'm not aware of any that are like this, however.