(1) Tailwind uses utility classes that feel like syntactic sugar over full CSS, so I don't feel like I'm "using Tailwind for layouts" as much as "using useful CSS grid presets"
(2) Much like with margins and paddings, text sizes, and colors, Tailwind helps me "pare down" the number of different values available to me. Much like having preset "m-1", "m-2", etc. values helps me be more consistent, having preset grid columns and gap spacing helps me stay consistent and not go too crazy.
(3) Because I'm using something closer to CSS-wrapped-in-classes rather than a set of components built by someone else, and because tailwind gives both a naming convention and an easy mechanism for adding my own classes that fit right in to Tailwind's, if Tailwind doesn't give me the exact values I want for grid spacing, then I can modify theirs. If I need to add a couple new presets in addition to Tailwind's, then I can do so easily.
(4) Early on when CSS Grid was new, I got bit by some bad layout bugs that were very hard to find, and impossible to fix (they were browser bugs). So, while I'm sure the bugs I encountered have since been fixed, I'm a bit gun shy on grids. I occasionally use Grid when it makes sense (via Tailwind), but more often than not I find it easier, more flexible, and safer to just use nested flexbox cols and rows. For which I also use Tailwind, if the project is of suitable size.
To sum it all up better, I view Tailwind more as syntactic sugar over CSS that helps me build my own design system. So, I would totally use vanilla CSS for small projects that don't need a design system. But for anything more than that, I like Tailwind for the same reason I like using web frameworks on the back end: it helps me keep things structured and consistent and more maintainable, even if I could do it all on my own without the framework.