Voidstones can be charged with Sextants to add a random enchantment modifier which applies to all Maps in your Atlas. There are 2 types of Sextants:

You can store Sextant modifiers you'd like to run later, or simply to sell them by using Surveyor's Compass directly on a Voidstone. This allows you to plan when to use specific Sextant modifiers, stack them or sell them to players that want to farm these specific modifiers. Some good high-value candidates are Delirium Mirror and Chayula Breach Sextants.

It is worth noting Map Sustain is greatly helped by Sextants adding monster packs to your Map. Also, most endgame strategies rely on specific Sextants to maximize profit. Finally, don't forget the great Enduring Influence notable on the Atlas Passive Tree because it lets you get more value out of your Sextants. Here is a list of all Sextant Modifiers: