A potential problem arises when generating an HTML index for caches, based on the "Placed by" entry.
For example, you might place some caches under one User Name (say, "Fred Nurk") then change your User Name (say, to "Team Nurk") and place some more caches. The two sets of caches will show up in totally different places in the index (even though they were set by the same person).
However, when you change your User Name, the internal (and unseen) Owner ID remains constant. Using the Owner Name forces all the caches for the same owner to be listed together. Clicking the Use Owner Name to group "Placed by" index check box on the Export HTML Files dialog box, ensures that all caches owned y a particular person will be listed together, regardless of whether they have changed their placed by Name.
On the other hand, you might place several caches in collaboration with another geocacher, in which case, these cache would be listed as being placed by, say, "Team Nurk and Snegvat". If you want all such caches to be listed together, don't click the Use Owner name to group "Placed by" index check box, otherwise they will be listed under the placed by name of the person/group who placed them.