One of the foundation of culture is transforming the land, starting from agriculture. The way the natural land was shaped by the people living on is a first marker of culture. Then comes the art and the buildings, in Europe we have a very dense network of historical remains spanning thousands of years, from cave paintings to modern art museums. If you have ever seen with your own eyes or touched with your own hands a famous remain of your place, you know the emotion is not quite the same as seeing it on the internet. It creates a feeling of belonging to a line of people who have created things bigger than them, and you reach a better understanding of how everything that allows your life today was progressively developed. It makes you want to protect this inheritance for others to access the same knowledge and emotions, and it may motivate you to produce something that could reach a similar value.

Land inspire artists and creators in general. The most evident example is how Mount Fuji inspired the creation of an insane quantity of art in Japan, such as the Thirty-six Views of Mount Fuji by Hokusai. The famous Great Wave off Kanagawa from this series, which is actually about the Mount Fuji, had a massive impact on the world culture.

Similarly, post-impressionist painter Paul Cézanne painted multiple views of mount Sainte-Victoire https://drawpaintacademy.com/mont-sainte-victoire/

The end of 19th century classical guitar piece Recuerdos de la Alhambra written by Spanish composer Francisco Tárrega is an interesting example. It was inspired by the Alhambra palace in Spain, which was built during the 13th century by the Emirates who conquered Spain during the previous centuries. Nonetheless, it became part of the Spanish culture and inspired new art pieces.
https://www.youtube.com/watch?v=fwjX-m4LkYk

To get back to the mountains, but with a historical scientific experiment this time (science is part of culture). Blaise Pascal did a historical experiment in 1648 at the Puy-de-Dôme (volcanic mountain in the middle of France) in order to test the "weight of air" that later led to the understanding of atmospheric pressure (immortalized by the Pascal unit of measurement). This would not have been possible in Holland (because it's flat). https://backreaction.blogspot.com/2007/11/blaise-pascal-florin-p-and-puy-de-d.html

There are countless examples of how ancient Greece and Rome inspired arts and architecture at different centuries: 16th century St. Peter's basilica, 17th century Versailles palace or Neoclassicism art in the 18th and 19th centuries.

There are many other examples about rivers, lakes, seas, cities etc...

That is to say, the land and how it is transformed and built are major sources of culture. If you take the same people but magically modify their history, so they lived on a different land, you will have a different culture. Probably not worse nor better, but different, because the land inspires the culture.

If your people are not living on its history soaked land anymore, you lose those major culture nutriments. I'm not saying people cannot carry part of their culture with them, nor that culture cannot reinvent itself and be inspired by different origins, but you have more chances to dilute or lose culturally important works of the past if they are not set in stone. Stones that you can access.

It may even get physically destroyed and erased, like what ISIL terrorists did of the ancient temples in Iraq and Syria. To be fair, it also happens from "democratic" movements, such as how the French Revolution destroyed many pieces of arts because it represented the Nobility and the Church. https://www.nytimes.com/1977/05/04/archives/notre-dame-statuary-lost-in-1793-unearthed-in-paris.html

https://eu.usatoday.com/story/news/world/2015/04/12/video-islamic-state-nimrud/25667399/

Given that Putin negates Ukrainian's history and culture as a justification to invade it, we can expect that he would continue on his pathway of destruction to erase any reminder of it.
https://theconversation.com/we-should-all-be-concerned-that-putin-is-trying-to-destroy-ukrainian-culture-179351

So that's a couple of elements that came to my mind about why defending your land is important for your culture.