The sealer coat handles some aspect of floor finishing that the finish or top coat does not. For example, waterbase finishes requires the sealer coat for two primary reasons. 1) so that you're not wasting expensive finish being soaked in the wood. 2) the finish is so strong that applying it without a sealer would generally result in the panelization and sidebonding that the other users have referred to.
If it is not a strong waterborne finish (like most catalyzed 2 components are) then you can use it for the first coat to act like a sealer coat (for example, we some time use Mega as the first coat instead of DTS or BonaSeal)
If it is not a strong waterborne finish (like most catalyzed 2 components are) then you can use it for the first coat to act like a sealer coat (for example, we some time use Mega as the first coat instead of DTS or BonaSeal)