Outsourcing firms might be savvy and helpful about both technology and industry expertise for inexperienced and confiding startup founders. This is how consulting companies gain trust and even dominance in negotiations and future business.
In my experience, every collaboration, when a buyer is in the submissive position end up with frustration and regrets for both parties. This happens because outsourcing partner will never replace inhouse personnel.
However, IT giants (Google, Microsoft, IBM...) are hiring more outsourcing firms and continue to expand remote teams. Seem like they knew something everybody else hadn't learned yet and chances are, most of the startups won't find out ever.
Moreover, for many reasons [wfh, quarantine, covid] hiring outsourcing agencies became more desirable and extremely popular in 2020.
I submitted some of my thoughts on this matter to the article How To Outsource App Development -- it is a good starting point for those who haven't ever tried to outsource engineering but already considered.