It works in that if you have a positive attitude towards something happening then you're more likely to take steps towards making that happen and/or take opportunities which you might otherwise feel are too risky.
Also, if you feel genuinely confident that something will work, then you'll usually perform better at it and present better to others, which is hugely influential. If you're anxious, then you'll pass that anxiety onto others who may otherwise help or support you. Actually I've found this is one of the most important things to understand in life.
It also "works" in terms of the Baader–Meinhof phenomenon - essentially what you're looking for, you'll notice more often. Like when you have a miscarriage and pregnant people seem to be literally everywhere when you wouldn't even have noticed them before.
It can be harmless enough in those contexts and if it helps you feel positive about things then go for it. In fact when it's framed as being confident, defining your goals and believing in yourself, then I think it's very positive. It's the magical thinking side of it which is a problem.
I have unfortunately seen a really toxic side to this belief system - have you heard the parable of the drowning man? Where the man in a flood is on his roof and refuses help from 2 boats and a helicopter because he has faith that God will save him, only to drown and arrive in heaven and ask God why he didn't save him and God says - I sent you two boats and a helicopter!
I've seen my mum get stuck in unhelpful thought loops similar to this, thanks to her belief in manifestation and other similar philosophies. Thinking that this opportunity which has come up is not the right one, because it is not exactly what she has "manifested". And while ultimately that is her choice, it hasn't worked out that well for her. She also seems to get worse (in terms of MH and general coping) the more she withdraws into this world. Whereas the more she comes out of it into the more tangible world she does better - from an outside perspective.
And growing up being told that thoughts literally create energy ripples in the universe which can cause good or bad things to happen gave me some really, really unhealthy worrying thought spirals as a teenager leading into adulthood, which (I was told by a psychiatrist) effectively amount to OCD rituals. It took me a really long time to be able to trust that I can have whatever thoughts I like in my own head and it's totally safe to imagine any scenario without that having any effect on the real world.