This way you avoid losing any of the combined lists. If you for example have a list of passed and failed you can easily create a complete list without losing the initial two linked lists. This is the same as if you would do:
String s = StringA + StringB;
In this case the StringA and/or StringB isn't lost because s is used to keep the result. This is way faster and shorter than:
String s = new String(StringA);
s.Concat(StringB);
Because of the use of the '=' sign it is very clear that there is data assigned to it. It is easier to spot than calling a method that assigns all the items to the object.
Good luck!
Ps. Maybe an extra consideration (cannot see into the actual .net code but I would optimize this way) could be that creating a new linked list with the results of both gives the opportunity to get all the linked list references scattered through memory and put them into sequential order. A linked list is a kind of list that is crafted to work without the need for that but of course would it help improve memory caching enormously. So this might then look like a waste of time but this simple copying once would speed up the look up actions and therefore , on average, worth doing.