As @brenthaag had mentioned, the DTH is used to allow ST to access the device features/communicate with it. The stock ST DTH only allows for basic communication, locking, unlocking and programming codes. The Enhanced Z-Wave Lock DTH fixes a few issues with the ST DTH, lock specific firmware bugs (e.g. Schlage FE599, August locks etc) and allows you to access additional features of your lock depending on your lock brand/model.
You can find a detailed list of features here:
[RELEASE] Universal Enhanced Z-Wave Lock, Schlage, Yale, Kwikset, IDLock, DanaLock, August Pro, Samsung, Locstar, KeyWe Locks and Popp Z-Wave Keypad Device Handler with Alarm Control, Notification, RFID, Door Sensor and advanced features
The SmartApp is used to manage/program codes for your lock. You can use SmartLocks or our free Single User Programming app for basic one off programming . For a more complete control of your lock codes including scheduling, custom actions, notifications, SHM/ADT integration etc you can check out the Lock User Management (LUM) SmartApp features here: (all the above apps work with the stock ST and enhanced DTH’s)