Mostly same as with any treesitter mode: it’s easier to build structurally-aware features, and it automatically supports any minor mode that’s enabled by treesi

Mostly same as with any treesitter mode: it’s easier to build structurally-aware features, and it automatically supports any minor mode that’s enabled by treesitter (examples from my own config include structural movement, folding, selection, indentation guides, etc.). So, rather than every language having to reinvent the wheel, and every user having to install six different language-specific packages to get feature parity, more stuff just works. You mention eg poly mode, which has always been extremely finicky for me. Syntax highlighting in treesitter modes is both more capable and more consistent.