1) This is a typical delta to wye configuration. I might add that normally these 3 transformers would be built as one chunk of steel with 3 sections, one for each phase. The floating Vn wire is just to show that the primary is delta only, neutral is not used.
2) Because this is 3-phase the difference in phases is 120 degrees. The 30 degree phase shift is the same in each transformer so X1/X2/X3 are also 120 degrees out of phase.
3) A typical application for this would be 480vac main feed from a service entrance panel to risers (buss-bars) going to each floor of a 5 story building. Each floor has its own 3-phase step-down transformer to convert 480vac delta to 120vac wye (208vac delta) to power the outlets and lights on each floor from a 3-phase 42 breaker panel. There is plenty of heavy machinery that uses 208 single or 3-phase for power such as machine shops, ultrasonic welders, etc. This is typical of what I did as an electrician so it is easy to write about.
4) The common neutral (shown)for X1/X2/X3 is grounded at its sub-panel, and though it is not shown a ground runs back to the main service entrance panel. In large buildings it is mostly a distribution panel for all the sub-panels on each floor.
5) If Va/Vb/Vc is 480 vac delta then the phase to neutral voltage is 277vac, typically used for lighting and HVAC blower motors and heaters. It is not unusual to have a double-neutral feed to such a building, to account for imbalances in the loading of the phases.
EDIT: For more details about the 30 degree phase shift between phase windings, please see this article:Why does a delta/wye transformer make 30 degrees phase shift ? and this article:What are the Pros/Cons on the 30 Degree voltage phase shift between the primary and secondary of a Delta-Wye Transformer
