Since first editions often include misprints, a particularly valuable source for urtext editions is a copy of the first edition that was hand-corrected by the composer.
The strictest possible practice is to render all markings literally, but an urtext editor may also want to point out the markings found in parallel passages.
The urtext edition adds value to what the performer could get from a facsimile by integrating evidence from multiple sources and exercising informed scholarly judgment.