From 百度：
"马甲，网络用语，出自2000年赵本山小品《钟点工》。 网络中意指在同一论坛注册多于2个（含2个）ID并同时使用时，常用的或知名度较高的那个ID一般称为主ID，其他ID称为马甲ID，简称马甲。"
To be honest, I wasn't aware the Chinese name 马甲 until starting reading in WXC.
In English, people with multiple IDs and use them on the same social media site are called Sock Puppets.
Per Wikipedia:
" A sock puppet, sock puppet account, or simply sock is a false online identity used for deceptive purposes.[1] The term originally referred to a hand puppet made from a sock. Sock puppets include online identities created to praise, defend, or support a person or organization, to manipulate public opinion, or to circumvent restrictions such as viewing a social media account that a user is blocked from. Sock puppets are unwelcome in many online communities and forums."
I have run into sock puppets before, once on a local housing English blog site, once a local Chinese site. Born to be curious and always in love with detective stories, I did some search, found the culprits' main IDs and exposed them with support of others on both sites.
For myself, I only use one ID on all social media sites other than WXC, and if WXC didn't reject that ID in 2021, I would have the same ID everywhere and would likily join MYSJ two years early in 2021.
So I have no clear idea why people need mutiple IDs other than the cases I run into:
The one on local housing site was to attack others while using her main ID for serious discusssion; the one on local Chinese site was to pretent a supporter of his main ID.
Maybe some people have multiple personalities so are in need to show off each under different IDs? Or just to fool around? Or maybe some need to see a shrink and check out their marbles? 
Care to share what you think? 
I know, I konw, all of us, including myself, are on the spectrum. But I never and will never ever have 马甲 for myself! 
Cheers!
