What plain text does and does not carry
A .txt file is a sequence of characters and nothing else. There is no font, no size, no bold, no margins, no page — those exist only in whatever program displayed the text to you. So converting text to PDF is not really a conversion; it is a typesetting decision, and the two choices this page exposes, the title and the body size, are the whole of it.
The one piece of structure plain text does carry is the line break, and it carries it ambiguously: a break might mean “new paragraph” or it might be a hard wrap left over from an editor set to 80 columns. This tool preserves what you give it rather than guessing, so text that was hard-wrapped stays hard-wrapped.
When a PDF is the right container
Reach for it when the reader must see what you saw: a document that has to print identically, be attached to an email without rendering differently in each client, or be signed. If the reader will want to edit the words, plain text or a Word document serves them better — a PDF deliberately makes editing hard, which is the point of it.
Going the other direction, PDF to Text pulls the words back out of a PDF, and HTML to PDF handles content that already has markup and styling worth keeping.