For a decade or two now, the ideology of multiculturalism has become almost cult-like especially in Europe. The media at large just can't shut up about it and everything related to it, politicians can't shut up about it, and it drives immigration policies, often to the detriment of the country and its people. Multiculturalism is taught in schools and in the media. It's always depicted as a good thing, this is basically always done in very vague, non-descript ways, without giving actual concrete reasons why it would be a good thing.
Curiously, and somewhat hypocritically, European multiculturalism seems to favor only immigrants of certain areas of the world, while completely ignoring, sometimes even shunning, people from other areas. The preferred areas are mostly Africa and the middle-East. Ignored and shunned areas consist mostly of rich western countries, Japan, as well as basically any country that is predominantly white, or that is predominantly westernized (such as most countries in the South American continent.)
The opposite is monoculturalism. Like multiculturalism, it can also be really extreme and pathological. However, if we don't go to the absolute extreme, I would argue that monoculturalism is significantly better for everybody than multiculturalism.
A nation prospers when its citizens work together. The less infighting there is, and the more people work in unison and understanding, the more that agreed rules and customs are the same, the more prosperous the society becomes, and the more it can progress socially, culturally, technologically and scientifically. Having a sense of camaraderie, even patriotism, having common values, customs and culture (assuming these customs are humane and beneficial to everybody, rather than being egotistical and greedy), helps with this.
When everybody shares the same values and the same culture, it decreases prejudice, distrust and hostility. It helps people work together, trust each other, communicate with each other, and work towards the same goals. The society will prosper when its citizens agree and understand each other, and want to work together.
Multiculturalism, in contrast, undermines all of this. Multiculturalists naively think that great diversity in culture, customs and values are a good thing, when it's exactly the opposite. The society becomes fragmented, and the amount of infighting, distrust, prejudice and cultural clashes will increase. Big groups of people with different values will not work together for the same goals. Valuable resources will have to be spent to placate to wildly different groups of people, all of them with different values and demands. People will distrust and hate people of other groups. Criminals will thrive because they may use their differences in culture as an excuse for their behavior.
This becomes especially bad when some of the cultures start demanding special privileges and special treatment, and when they look down their noses at people of the other groups. Social unrest and crime will raise, and more and more resources will need to be spent to keep up this fragmented society. The society will become restless rather than peaceful.
Note that monoculturalism is not inherently against immigration. Immigration is fine, in moderation, but monoculturalism requires that the immigrants adapt to and respect the hosting culture. The more same values they adopt, the better.
There are no practical benefits to multiculturalism. It's just a naive ideology. An ideology that makes some people feel better about themselves. An ideology of virtue signaling. In contrast, there are plenty of practical benefits to monoculturalism.