Well, first of all hi to everybody, i'm a newbie in this cryptocurrency thing, so a lot of doubts are in my mind these days.
I have this question, looking here for the answer I didn't found yet, basically the subject says it all.
If bitcoin transactions are irreversible, why some platforms (online wallets, exchanges, etc) need X confirmations to "validate" a transaction?
You answered yourself. Exactly due to fact that bitcoin transaction are irreversible it is wise to wait for at least 1 confirmation of the network.
Unless there is 1 (or more confirmations) there is a risk of 'double spending' of bitcoin.
Even with 0 confirmations the risks of double spend are insanely overblown. Unless you are paying a lot for something really important, then chances are you can be perfectly ok with 0 confirmations and sleep better at night, certainly better than credit card transactions.