Generally speaking, the OnActivate event needs to be used anytime you think that you need to use the Form_GotFocus. That's because a Form can only receive Focus if there are no Controls on the Form that can receive Focus, which is a relatively rare event!
Typically, OnActivate is used in situations like the OP described...where the user navigates from one Form to another Form, then back to the original Form, which is already opened. Also typically, it involves situations where you've added data to a Table, via the second Form, and want that data reflected in the original Form. You can use it to Requery a Listbox, a Combobox or the entire original Form.
And, yes, given the number of times I've had to recommend it, for doing these kinds of things, over the years, it probably is underutilized!
Linq ;0)>
The problem with making anything foolproof...is that fools are so darn ingenious!
All posts/responses based on Access 2003/2007