Maximum damage

A cold facet will increase your base cold dmg by the stated 5% as additional cold dmg.
So say you're using a cold skill that does 100 dmg, you have 20% +cold dmg from other gear, you now add that 5% facet, you end up with 100 * (100% + 20% + 5%) = 125 dmg (before resistances).
A cold facet's % dmg only ever increases cold damage. It will not for example take your e.g. 100 physical weapon dmg and turn them into 100 physical + 5 cold. It'll just not do anything there.
The 24-38 cold dmg on your facet on the other hand generally don't do anything for non-weapon attacks so that doesn't get added to your blizzard or whatever.

A max dmg jewel, much like an enhanced damage jewel, is the physical version of that, increasing your physical base dmg by the stated 14 which is then multiplied out by your physical %dmg multipliers.
It will not add any damage to cold skills though.

So yeah, if you're say a cold sorc and looking to improve your blizzard or orb or whatever, a cold facet will up your damage there but a max dmg jewel won't do anything.
If instead you're say a pally and looking to improve your

Zeal dmg then a max dmg jewel would help you while a cold facet would generally do precious little (it would still add the 24-38 cold dmg to your attacks).

Do note that the physical gain from a max dmg jewel doesn't affect all physical dmg abilities/sources but is generally limited to weapon-based attacks. E.g. it'll help

Zeal but it won't help

War Cry or

Smite.