Group elements for selection purposes — Don't let them get in your way, though.
XD groups vs Select Groups
Select-groups allow more flexibility while working on elements that are not entirely related to each other and are only required to be selected together on occasions.
For example — If you need to select all the title texts on all the artboards in a project, but you do not want to attach them as a regular XD grouping does, you can select-group them. This way, you do not have to double-click on each one when editing them separately, and if you need to change them at the same time, you can select them at once by clicking on the select group.
Add to multiple groups
Unlike the regular groups in XD, in which you can add an element in only one group, you can add elements, from multiple regular groups, in select-groups. This can come in handy if the element/s is part of various groups of elements you want to select on several occasions during the design process.