There is some sense in it, in that whatever you're focusing on, you tend to unconsciously aim towards. If you feel secure, confident, happy, that you have a solution to all of your problems, then you will also act that way and acting confident tends to lead to better outcomes. This is no more than a psychological trick similar to positive thinking. It works because it encourages you to move towards things that you want/need/would like rather than shying away from them and assuming they aren't possible or won't work for you.
However, I will tell you the negative/toxic side of it. My mum is really into it and got me into it as a teenager. There were various stories which all seemed to prove magically that it worked, which were probably no more than coincidences/an effect of the "focus on what you want and you'll orient towards it" aspect. And I'm sure that there were positive aspects for me, for example pursuing a course in the direction I wanted to work etc.
But it also caused me (among other things, but I'm sure that manifestation was one of the mindsets that was leading me here) to stick around in relationships - romantic and friendships - that were unhealthy and toxic for me. Because manifestation led me to think that I could influence their behaviour ~on an energy level~ rather than learning about boundaries and how to communicate and enforce them. Now I'm sure this is a common teenage thing and if you're coming to it as an adult, then perhaps it's less of an issue, but I wanted to share my experience as you do not always hear of the downsides.
Secondly, because of the idea that "thoughts become things" there is sort of an opposite vein too, an idea that you mustn't ever think about anything bad that might happen, because thinking about it can literally cause it to happen. This can be pretty majorly disastrous for mental health, especially for anyone who is imaginative or a worrier. It probably was helpful for me to be told "don't focus on all the things that could go wrong", but OTOH, it was not at all helpful for me to become afraid of my own mind or get into these sort of mental "rituals" where I would briefly worry about something (like a house fire, or a car crash, or something happening to someone I love) and then worry that I can't push that thought away, and have to resort to trying to block it out by thinking WE ARE SAFE WE ARE SAFE WE ARE SAFE WE ARE SAFE over and over again, to try and block out the thought. This escalated almost to the point of OCD level thoughts, where I really believed that I had to keep thinking this, otherwise I would CAUSE something bad to happen. That was extremely unhealthy for me and caused me much distress. It took quite a long time and lot of effort to be able to retrain myself that my mind is actually a safe space to explore absolutely any thought that came into my head, including disaster scenarios or bad things or scary things, and thinking about it absolutely cannot possibly in any way cause it to happen. In fact, being able to think about the ways that things might go wrong is OK, as long as it is not obsessive; it's a healthy and helpful way of risk management. Manifestation theory says that it's not. And this was unhelpful to me.
Thirdly, I've seen my mum get more and more and more into this magical thinking to a point where she doesn't want to make any steps to actually make things better for herself because she is waiting for the "most perfect thing" to come along and she won't do anything unless it "feels right" in her gut, which leads her into some precarious situations, which she doesn't like to acknowledge because (as she puts it) "that is just going into fear, I don't want to go into fear, I want to go into love". Unfortunately she's been relying on this for probably the last 20+ years and it hasn't got her anywhere. This is immensely frustrating and difficult to watch as an outsider and I know that it hasn't made her happy either.
If you look at the people who are selling this stuff they are making a huge amount of money from it. It's not just out of the kindness of their hearts. They know what they are doing and it's a manipulative venture just like many others. If it was harmless or net positive then I might say fair enough, but unfortunately I know enough of the darker side that I am highly cynical about the whole thing. There are actually aspects of a high-control group about a lot of this, especially when you look at a lot of the adjacent topics (spirituality, love-not-fear, this idea of energy being some kind of mystical force, faith or "energy" healing, lightworkers, starseeds, reject the "mainstream" etc.)
There are useful aspects to take from the idea, but please do not buy into the notion that "universal energy" is just waiting around for your intention to direct it into things. This is total nonsense and where all the harm comes from.