Benefits to maxing out Grand Company's

[Zidaryn]

If you are asking about the usefulness of the different items you can get from trading in your Grand Company Seals, it depends on what you're doing. For me, I "Expert Delivery" a lot of the useless loot I get from doing dungeons to get Grand Company Seals and I buy Ventures with them for leveling my Retainers.
Also there are Survival Manuals for leveling your Disciples of Land/Hand faster. There's also a bunch of crafting materials and you can get the armor set to use for glamour.
There's nothing much that you really NEED... but some of it can definitely be useful, or you might like it.

[Mendalas]

If you are asking about the usefulness of the different items you can get from trading in your Grand Company Seals, it depends on what you're doing. For me, I "Expert Delivery" a lot of the useless loot I get from doing dungeons to get Grand Company Seals and I buy Ventures with them for leveling my Retainers.
Also there are Survival Manuals for leveling your Disciples of Land/Hand faster. There's also a bunch of crafting materials and you can get the armor set to use for glamour.
There's nothing much that you really NEED... but some of it can definitely be useful, or you might like it.

To add to this, you can also buy items that teleport you right to your Grand Company headquarters for free. There is also food that you can buy with company seals that is pretty decent for levelling dungeons and crafting/gathering

[DrWho2010]

Access to items for purchase for crafting and gathering and staying with your grand company of choice works your way towards mount rewards in frontlines PVP.