Just guessing here, but by introducing a CTA between the CTR and the TMA they may have had to add an additional layer of ATC as well.
The CTA is usually controlled by the same people as the CTR, it seems to be purely how you label the block of airspace.
for us uncontrolled VFR flyers the difference between a CTR, CTA and TMA is really moot.
I think that's true for IFR as well, its only the airspace class that makes any real difference.*
*I think ATZ and RA are the only airspace types which have any real requirements. The rest, DA, HIRTA, MATZ etc. are just 'hints'???