Usually because they need the technology the vendor is selling.

But buyers try to insert this language into partner/ biz dev contracts all the time.

Much less common for sales.

100%.

A place I worked some years ago we even had an escrow foisted on us by our larger partner in the agreement so that they’d be able to continue running the software we were building if we went under.

Honestly, it was a pain in the ass and meant that for them alone we ended up running an older version of the software than we offered to clients because as we developed its capabilities it became ever more integrated into our core platform and we weren’t about to escrow that.

When the agreement came up for renewal at the three year mark we managed to get the escrow clauses removed.