For a field rifle, there is no use in my opinion...just adds extra weight, and extra moving parts.
For target use, it definitely has a benefit (accuracy)...and yes, it is good for shooting at less than 100yds. Rimfire and airgun junkies often require their adjustable objective to focus down to 10-15yds.
The adjustable objective gets rid of parralax.
I've found that many of the people shooting scopes with A/O's don't even know how to properly use them.