A sealer isn't any more "compatible" with the old paint than anything else, in fact it is thinner to lay out better so being it has more solvent in it it is more likely to lift something or fail in some way.
Sealer is used for a number of different reasons.
1. Poor prep, too course of sanding for instance.
2. Make a uniform color for easier coverage with your paint.
3. Small sand thrus.