Why we should not lvl up gems?

Hi.
When i started this game I did not know anything (started at OB) and while leveling I level up all my gems.

But then I read everywhere we should not lvl up gems. Can someone explain me why? I was sure higher lvl = more dmg, better buffs etc.

So any advice which one are safe for leveling and which one we should not lvl up?

certain skills, according to half of the players, don't get much benefit from level ups.

the ONLY reason not to level up a gem is for mana cost or if you want to later give the gem to a newer character.

edit: oh and i'll add in my opinion and more specific examples lol. detonate dead gem only increases the physical damage part of it (and mana cost), so there "is no point" in leveling it up although still a point to use it. clarity (and i guess the other auras too) has large mana cost jumps between level ups, so hold off on boosting it if it'll just take most of your pool.

As soul4hdwn said, some skills, especially auras increase mana cost by quite a bit per level, which sometimes you can't afford or you don't need the benefit of right away. Normally you'll always level a skill up unless you have a reason not to.

For example, Let's say I have 200 max mana, Clarity running at 160 reservation and 35 Mana per second. If my main skill costs 30 mana to cast once per second, and Clarity is ready to level which will increase the reservation to 180, so I wouldn't want to level it because first, I wouldn't be able to use my attack skill, and second, it doesn't serve much purpose since I'm currently regenerating more mana than I use per second.

Another reason not to level them is the stat requirements; say your armor gives you 20 dex, and a skill gem that is ready to level requires only 5 less dex than what you have, you'll probably level it up because hey, you have the needed requirements! But wait, you just remembered you have an armor piece in your stash that you were going to use next level that doesn't have any dex bonus on it; at this point, you can choose between leveling the gem, and being unable to use the armor in the stash without finding another dex source, or wait on leveling it, equip the armor next level, and find a new dex bonus without sacrificing gear.

Then there's skills like Detonate dead, as soul4hdwn mentioned, which does you no good whatsoever to level up if you're just using it to prevent Resurrections.

But as a Summoner Ice witch should I lvl up summoning gems (like zombies etc)? Higher lvl = more hp but I dont know now.

Check poe wiki to see how certain gem level up, what stats are being changed, mana cost etc and you can easily (or not) judge if it's worth it ;)