There are investment management and financial planning firms that only work with professionals with professional corporations.
At one time, when I was a licensed advisor, I worked for one of those firms, specifically on the tax and financial planning (not investment) side. As has already been alluded on this thread, this is largely a tax planning issue.
It is also a financial planning issue writ large, as the professional structures income, holdings and the practice from active ownership and management to different ownership structures as they transition to retirement.
This is an area where I think professional advice (not money management advice, although the initial question was about investment holdings) is warranted. There are a lot of factors which influence what the right structure is for any one individual and any one professional corporation.
(This isn't a very useful post. I don't think it is appropriate to make general statements online on this kind of topic.

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