I'm about to sign up to an online marketplace and they give this as an option for me as seller.
Yep, we do this as an ecommerce platform provider.
Firstly, Stripe is a good platform and has a wide range of services from ID verification and anti-fraud tools. So would recommend.
As for Stripe Connect; this can vary depending on the ecommerce/marketplace provider and how they operate.
For us, we have all options set for the merchant to deal direct with Stripe and all settings, etc, are under the merchant control and between them and Stripe. What we get out of it is access to connection logs, transaction details and the sort of stuff from our clients Stripe accounts that enables our support team to better support our customers in the event of any technical issues.
On the flip side, my daughter uses a platform built for her industry where the Stripe Connect partner receives all payment funds, handles payments, and passes my daughters 'revenue' once a week for what she has received in booking payments. In that scenario my daughter has no direct relationship with Stripe, but it works for her. For me in my business, I didn't want that responsibility and opted for the route we did purely for a tech support perspective.