Leveling skills in Elder Scrolls Online

What exactly is the point of leveling a skill? I sure do get that you can unlock new Skills and perks but does It do something else? Will you gain more Armor/Damage if you have a Higher rank in a certain skill in ESO or is it solely for the sake of unlocking new stuff.

Also if you Level up an Active Ability, will this ability get stronger or is it also just for upgrading an ability. If it's only for upgrading, then why can I further level the upgraded skill (I've noticed that you can upgrade an Ability only once.

Another thing I'd like to know is: What is the max level of an Ability/Skill?

See if I can answer all of your questions.

What exactly is the point of leveling a skill?

To gain access to higher level skills.

I sure do get that you can unlock new Skills and perks but does It do something else?

No.

Will you gain more Armor/Damage if you have a Higher rank in a certain skill in ESO or is it solely for the sake of unlocking new stuff

Yes. Not sure how much it scales.

Also if you Level up an Active Ability, will this ability get stronger or is it also just for upgrading an ability. If it's only for upgrading, then why can I further level the upgraded skill (I've noticed that you can upgrade an Ability only once.

Yes. Not sure how it scales though. I believe they are planning on implementing one more morph to the skill lines in the distant future.

Another thing I'd like to know is: What is the max level of an Ability/Skill

Rank IV > Morph Rank IV. Max Level of weapon/armor/class skills is 50.