throne = grace + mercy

Let us then with confidence draw near to the throne of grace, that we may receive mercy and find grace to help in time of need. - Heb 4:16

This is such a rich passage. Read it slowly.

The throne of grace is to be a frequent visiting place for the believer, for mercy when we sin, but also for grace to help us not sin.

Calvin on this passage:
The ground of this assurance is, that the throne of God is not arrayed in naked majesty to confound us, but is adorned with a new name, even that of grace, which ought ever to be remembered whenever we shun the presence of God. For the glory of God, when we contemplate it alone, can produce no other effect than to fill us with despair; so awful is his throne. The Apostle, then, that he might remedy our diffidence, and free our minds from all fear and trembling, adorns it with "grace," and gives it a name which can allure us by its sweetness, as though he had said, "Since God has affirmed to his throne as it were the banner of 'grace' and of his paternal love towards us, there are no reasons why his majesty should drive us away.