A flip on the incentive might be that their core product uses open-source software somewhere in it and the style of open-source license may compel them legally to release any modifications, additions, etc. I don't know which licenses that may or may not apply to and IANAL.
Also for reference, RedHat's entire business model was like this, mostly charging for enterprise support. I use past tense because I have no idea what they do today. Way back in the day that mean when you submitted a support ticket, you might get a developer that worked on the product to help you if it was beyond their initial support triage level.
More sales and trust. Vendors who open source are offering their customers the right to repair and self-host their products should they go belly up. That itself is a selling point.
They can't compete with the closed-source ones. Open source is a very efficient user acquisition strategy. Not as much to turn users into paying customers tho. But it is a way to go from 0 to 1 in a market that otherwise would be very difficult to call attention.
Also for reference, RedHat's entire business model was like this, mostly charging for enterprise support. I use past tense because I have no idea what they do today. Way back in the day that mean when you submitted a support ticket, you might get a developer that worked on the product to help you if it was beyond their initial support triage level.