It normally means that they have detected something that links the poster to a previously banned poster, or someone has name changed and their two personas contradict each other etc.
As to why it matters, well there's a few reasons.
People generally don't want to give advice to someone who's lying about their situation.
Someone for example trolling that their child has died -( this has happened more than once). It's not harmless. It deeply hurts bereaved parents. There are lots of these. Dead or dying children. Violence. Cancer. Making up shot and seeking support from people who have actually suffered is grotesque.
Those posts can't stay. It's nothing short of mockery.
Trolling with financial sob stories trying to dupe people into sending cash is, well, it's fraud isn't it? I've got 37p to last a month, I need to feed my kids something, does anyone know if toilet paper is toxic? - ok, I've taken the piss there but if I had the sum total of all the pennies people have claimed to have left to feed their children for X amount of time, I'd be a millionaire.
Those posts can't stay. The more MN is known to be a place where people give money to strangers, the more con artists will appear.
Christmas appeal remember? It got so I'd put my decorations up the first post I read from a new poster saying they've got no money does anyone know how they can get money then explaining why each suggestion wouldn't work. Oh gosh it seems only cold hard cash will help you. Fancy that. Really got me feeling festive.
££ - Advertisers. Troll central is far less attractive to companies trying to sell their shit to parents.
A zero tolerance rather than assessing each pile of bollocks to see if it's amusing enough to stay discourages people from trying to make it into the funny hall of fame.
Mns just too big for that these days. When it first started it was small and fun and a lot of pissing about but it's a business now and it's not good for business to have boards full of trolls.