Online friends have a social anonymity that real life friends do not. It is easier to both judge and be judged as an individual, rather than any existing social categories that we might fall into. Relationships can begin without any potential bias - we need not know our friend's age, gender, location, physical or mental health, wealth, class, social groupings etc.
The anonymity also means that if the relationship was to sour, the ability of either party to cause social harm to the other is minimised. This means that being more honest/sharing more 'secret' information is comparatively low-risk, and so sharing is more likely and, subsequently, forming deeper relationships is safer and in some ways easier.
Similar interests in video games, tv shows, ect. Also, I don't completely feel this way anymore, but there was a point where I felt it was easier to meet a friend online, especially for people like me who had trouble making friends. I've noticed that when meeting someone online, sometimes you can mostly skip the small talk and just tell them whatever's on your mind, which facilitates closeness.
Much easier to find friends sharing specific interests
Commonalities in interests, identities
We have similar interests, and we are struggling with the same things. As a young LGBTQ person in a country that is very against our existence in real life not many people are open about it and we can't talk about it there as much, so we talk about it with our online friends, and they are some of the most supportive people I know, and they don't ask inappropriate questions like my offline friends.
They talk openly and correlate more with my interests and hobbies
Online friends tend to be more accepting, trustworthy, fun and open-minded in general.
We enjoy some of the same, more niche things that my offline friends don't. We also have similar personalities and have similar circumstances, like chronic illnesses or traumas.
Common interests that aren't affected by distance
Cultural differences, I have friends from very different countries
Easier to make friends with, more casual of a relationship, less pressure to be or do something, no follow-up obligations to attend things or make face time because of distance.
It's easier to talk to online friends sooner about serious topics like illness, mental health, and relationships.
They will keep secrets
They are a separate group from my irl friends meaning that anything I say to them will not be spread to my irl friends to effect my reputation
I have online friends from all over the world, so I can pretty much guarantee that there’s going to be someone to talk to whenever I’m online
Memes
Ability to discuss things too taboo or personal to discuss offline, no pressure to meet up irl
some anonymity and freedom from judgement or expectation
maybe by not judging me based off appearance alone or who i might associate with in real life or what they hear from other people solely because of the fact they don’t know me in that aspect
Physical contact/presence