LaTeX mode gives you the raw source in a real code editor, for when you want full control over packages, environments, and macros. It is the right surface for fine-tuning a preamble or hand-writing a complex table.
The source editor
The editor has syntax highlighting, bracket matching, and command autocomplete. Start typing a command and Writano completes it; common structures are available as snippets so you are not retyping boilerplate.
\begin{equation}
H = -t \sum_{\langle i,j \rangle} c_i^\dagger c_j + \text{h.c.}
\end{equation}
The outline panel tracks your \section and \subsection structure as you write, and the ⌘K command palette jumps you anywhere in a long document without scrolling.
Preamble and packages
Because compilation runs on a complete LaTeX distribution, the packages you would expect are already present — amsmath, biblatex, tikz, graphicx, and the rest — so \usepackage in your preamble just works. There is nothing to install locally.
Same document, different surface
LaTeX mode edits the very same document as Visual and Markdown mode. Toggle to Visual to see the rendered result, or stay in source and press Compile (⌘↵) to build the PDF. Your mode choice is remembered per document, so a paper you prefer to hand-edit keeps opening in LaTeX.
Note: SyncTeX links the source and the PDF both ways — click a line to find it in the output, or click the output to find the line.