Hi Ari and Leesee,
The thickness of a mattress is just a side effect of the design and by itself isn’t particularly meaningful, because whether a thicker or thinner mattress would be better or worse for any particular person will depend on the specifics of the materials (type, firmness, etc.) and on all the other layers in the mattress. Thickness is only one of many specs that are used to make different mattresses that perform and feel differently and that makes a mattress suitable for one person and not another. There is more detailed information about the effect of thickness in post #14 here. Regardless of how thick or thin a mattress may be … the most important part of the “value” of a mattress is how suitable it is “as a whole” for your particular body type, sleeping positions, and preferences in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) regardless of how thick it may be.
As for latex, the main benefit of a thicker latex mattress (or any mattress that uses similar materials) is that it can be more adaptable for heavier weights and multiple sleeping positions. It will compress from softer to firmer more gradually which means that there is more “range” of compression without the mattress becoming too firm for heavier weights (or parts of the body). A thicker mattress can also be part of a specific design that requires it such as some types of zoning or layering that needs more layers to accomplish the design goal of the mattress. It can even just be a matter of preference rather than “need”.
If you can imagine for example a 2" layer of latex (or any foam material) on the floor you would compress it to it’s maximum and feel much more of the floor (it goes from soft to maximum compressed firmness within the space of a 2" layer) but if you had the same softness of latex in a 6" layer on the floor … it would feel much softer and compress more gradually and to a lesser percentage of its overall thickness and wouldn’t reach the same level of firmness or “bottom out” (which means reaching the maximum level of compression or firmness that a layer or a mattress can effectively achieve).
In the large majority of cases … 8" - 9" of latex is easily enough to include the combination softer layers (or sometimes sections) for pressure relief and firmer layers for support that most people of average or even higher weights would need. In some cases, people that have much higher body weights or larger body types may choose more than the “average” thickness and may prefer the feel and extra adaptability of say 12" of latex. Thicker mattresses can also use firmer materials because thickness and softness are very related and work together. These are all good questions to ask the manufacturer or retailer of a mattress so they can discuss the differences between the different options they have available in terms that are more specific to the layers they use or options they provide.
Phoenix