GE supports association for one group, the Linear master dimmer does not support association (although some of their original marketing materials said they did). The Linear auxiliary supports association for one group (used to create the virtual three way).
That’s most significant if you want to create a situation where physically changing one switch causes things to happen with other devices.
You can always Do that if the instruction to change the first device came through the hub, like from a routine, or even just toggling the device in the smartthings mobile app.
But if you start from the actual physical switch, it will work a little faster if that first switch supports either “instant status update” ( like the Cooper and the more expensive Leviton line) or supports “association” like DragonTech and GE. because the Hub find out about the change at the switch faster.
many households don’t need switches that support zwave association
For some people, association being missing as a feature won’t make any difference at all, because they never have the situation where they want to physically flip one switch and have other devices besides the ones on that circuit come on. But for others, association is very useful.
(I should also say that even if we’re not using the association to connect directly to those other devices, we might be flipping a switch on a Z wave and wanting a Zigbee device to come on. So Instead the association is used to send information from switch to the SmartThings hub. Rather than waiting for the SmartThings hub to get around to checking in from time to time. So we would have a rule for SmartThings that when the switch is flipped physically, something else should happen. Association between a switch and the hub or “instant status” each just let us tell the hub that the switch was flipped a little sooner. )
Three way setups connect one light to two switches. Zwave Association connects two devices directly to each other.
So three-way set up, whether it uses traveler wires are not, lets you turn on either of two switches and have the same light/lights come regardless of which which one is used.
If the connection between the two switches uses physical traveler wires, there’s a limitation on where you can put the switches. Typically one on each end of the hallway for example.
If the connection between the two switches is “virtual” (no traveler wires), you can put the second switch anywhere. 
That opens up a lot of possibilities, which is why most manufacturers now don’t use the traveler wires for network switches.
“Association” lets you have one device control another device wirelessly. It can be used to set up a virtual three way. Or it could be used to have a motion sensor trigger a light switch or a contact sensor trigger a siren. Or it can be used to let the switch inform the hub when the switch is changed manually.
“Instant status” is a different feature which also lets a dimmer switch notify a hub when the switch is changed manually.
And there are indeed switches, like the Cooper’s, which actually offer all three kinds of options: no traveler wires, support instant status update, and support association.
check each model, not just the brand, for the features you want
Check each model carefully. Supporting “instant status update” will make a switch more expensive, so Leviton, for example, has two different model lines. I believe both support Association, but only one line supports instant status update.
And, as we started with, the GE switches are inexpensive switches that do support association. The DragonTech are inexpensive switches that support association for one group (which is all you need if you just want to set up a virtual three way). The Linear are in the next group up in terms of quality and price, but most models don’t support association.