- It's a major transition point in abstract art, moving from painting that still have an echo of representation to the purely abstract. You might not like it, it's still an important part of art history. It's talking (with fairly sophisticated language, if you're able to listen) about art in and of itself. About the removal of nuance in favor of structure. About the use of white space.

- It influenced and shaped modern architecture, via Bauhaus and "International Style" skyscrapers. That painting (and Mondrian's approach) shaped every major modern downtown, to some extent.

- It's the precursor of the entire field of graphic design.

Again, you may not like it. Art has always been in the eye of the beholder. You may dislike what it says. But it does say a lot, clearly and in a well thought-out way.

It says even more if you see it in the context of his earlier tree paintings. For folks who care, a rough sequence:

- https://en.wikipedia.org/wiki/Evening;_Red_Tree

- https://www.piet-mondrian.org/the-gray-tree.jsp

- https://www.piet-mondrian.org/the-flowering-apple-tree.jsp

- https://www.moma.org/collection/works/79816

(Most art gains if you see it in the larger context of the artist's work. Most modern art exhibitions fail to make that clear)