Product review (5 stars): You don't NEED it; but you may WANT it,
I'm going to start by echoing what others have said - in most cases you will not NEED this adapter. It's really all a matter of what this does and whether you WANT this adapter.

Any new disposal will come with a mounting system so that you won't NEED this adapter. If you want to keep your old 3-bolt mount in place and your new disposal is NOT a 3-bolt mount (EZ Mount, for example) you will WANT this adapter.

We got a new Waste King Disposal. EZ Mount. The new flange is chrome. All of our kitchen trim (including sink drains and faucet) are brushed nickel. So, for this reason, I WANTed to keep the existing fixtures in place and that was a 3-bolt mount.

Also, as this was already mounted, puttied, leak-free and happy, I decided it would be much easier NOT to re-do the mount. Hence the WANT for this adapter.

Given that, it was very easy to install on the disposal (in place of some parts of the EZ Mount system) and made installation of the new dispsal as simple as rotating the lower flange into place, connecting the dishwasher and waste plumbing and plugging it in,

So, you likely don't NEED this. But you get to decide, based on some of the considerations above, whether you WANT it.