From Diablo Wiki
Whether your a Paladin, a Necromancer or an Assasin, All Skills will boost thier skills by a said amount. Although it does increase all skills by 1, it can only increase skills that have been activated with a clvl skill point. If I was to say want to add Bash as my new skill with a "+1 to All Skills", instead of bash already being 1 from the start, i would have to use a skill point to activate the +1 to All Skills to the bash so the end output would be that i activated it with 1 point and the slvl of the Bash is now 2.
This is a very good enchantment for boosting up any long-awaited skills and to exceed already maxed out skils to a higher potential. Any class with a skill tree has a great use for this enchantment, although it is rarely found on any common item and is mostly found within the handfuls of set and unique items. This is also good for diversifying with any other classes you might have, so to say that you don't need to find another item that will benefit the class directly, such as a "+1 to necromancer skills".