The purpose of shield is to fill your RA with your OA instead of your SA first.
You can shield OA dollars too if you wish. For example:
1. Shortly before your 55th birthday, use the CPF Investment Scheme (SA) to shield all but the minimum ($40,000) that must stay behind in your Special Account.
2. Shortly before your 55th birthday, use the CPF Investment Scheme (OA) to shield all but the minimum ($20,000) that must stay behind in your Ordinary Account.
3. On your 55th birthday, the CPF Board creates your Retirement Account and sweeps $60,000 into it ($40,000 from SA and $20,000 from OA).
4. You then top up your Retirement Account at least to the Full Retirement Sum (presumably), or optionally as high as the Enhanced Retirement Sum.(*)
5. You lower both shields.
Variations are possible, but this is one example. This "double shielding" example results in an even bigger pile of Singapore dollars spread across your Retirement, Special,
and Ordinary Accounts. A cautious, conservative saver/investor might rationally execute this maneuver since 2.5% interest earning OA is decent these days, and to enjoy the unique asset protection benefits CPF offers. It's otherwise more difficult to stuff as many dollars so quickly into CPF.
In other words, with this maneuver you're shielding the maximum number of SA+OA dollars you can from the entire sweep into your RA at its creation in order to maximize the RA top up then available just after the sweep.
This maneuver is for fairly well off individuals, of course -- individuals who don't need to raid CPF assets at age 55 and who have spare assets (cash really) lying around that are earning less than 2.5%/year and/or that the individual wants to protect from certain creditors and court judgments.
(*) I think technically you can make a sufficient property pledge or have a sufficient property charge and push your RA only up to the Basic Retirement Sum, leaving the rest of your CPF SA+OA unencumbered for on demand withdrawal. However, that seems a bit silly in these circumstances since the usual default is you'd still have ~$60K available for RA withdrawal any time you wish since that's what got swept in from SA+OA.