i dont rly understand the point of a charm like this... do people use this in their charm slot and put their offensive high ee charm in the secondary charm slot??? dont the charms in the secondary slot do like 50 or 75% their damage ?

Yes thats how it works, a high int charm with some other nice mods like mana or mastery in main slot and some high ee in the acc slot.
The EE in the acc slot is limited (I think 75% is the correct value) but you get more profit from the int and the other stats in the main, at least in the higher lvls where u have like 50+int on the charms

most people that do casting on 2nd slot dont have enough mana to cast through the complete lvl (for example offclass caster)
so they try to compensate a little for the big dmg loss of the 2nd slot and increase their amount of mana. So they do 25% less dmg but also spend 25% less mana per cast. So the dmg per mana gets increased if you have an int charm in your main slot but still the dmg will be lower.
on the lower lvls especially on offclass casting it can be useful because of the low mana. on the higher lvls you will use your dmg charm on main slot and healing on 2nd slot unless you are the main healer on high lvls and need a stronger heal (then you heal main slot)

If you need a heal before level 20 you’re doing it wrong. I don’t particularly like shrining

Wow that wild heal is awesome

Stingray and ahs pointed out the most common factors to using it.

At level 1 to level 5, you can put a 55+ % Enhanced Effect Cast charm in Acc slot and still 1 to 2 shot kill mobs (depending on type of element)
At level 1 and +200 Maximum mana increases your Mana Pool equivalent to adding 100 intelligence (+2 mana pool per Int point @ level 1, as you increase in levels, it goes down by 1 or 3, haven't tried to work out the mathematics)

Off cast or not, this charm can aid a caster from level 1 to 15 before switching if you don't have a better one in between that time.

Again, it is up to the user, for Cheeya, he uses Fire, so the 2% Fire Mastery is also a benefit.

Sorry for my pedantic type of reply, but hopefully it helps answer your question.

okay, so its a question of mana pool being too small then, got it !

Magical fine alloy dagger
Level Req: 20
Damage: 74 to 130

+700 Max Mana

Perfect drop

Ep at lvl 71

Magical steelwood staff
Level Req: 45
Damage: 137 to 366

+153% Enhanced Effect

very nice!