One reason I could see is if upgrading in place is substantially faster or cheaper than migrating data to new ones. For instance, if the nodes are storage node

One reason I could see is if upgrading in place is substantially faster or cheaper than migrating data to new ones. For instance, if the nodes are storage nodes in a redundant storage system, taking each one offline briefly for a reboot, and letting the others handle the slightly higher load, is a lot quicker than spinning up a new node and replicating all of the data over there so you can de-provision the old one. Also, even if they don't have a lot of data, the time and resulting expense of spinning up and provisioning a new node while the old one is still online could add up to higher costs than just performing a reboot during a point of lower load.