When should I use setDT() instead of data.table() to create a data.table?

Question

I am having difficulty grasping the essence of the setDT() function. As I read code on SO, I frequently come across the use of setDT() to create a

Answer 1

Update:

@Roland makes some good points in the comments section, and the post is better for them. While I originally focused on memory overflow issues, he pointed out that even if this doesn't happen, memory management of various copies takes substantial time, which is a more common everyday concern. Examples of both issues have now been added as well.

I like this question on stackoverflow because I think it is really about avoiding stack overflow in R when dealing with larger data sets.