Many presentations degenerate over time. What started fresh, clean, and straight to the point gets diluted over time.
More people start editing the presentation. They do not understand all the slides 100%, so they add bubbles and bullet points with text just to make sure that the point gets made somewhere in the presentation. Bits of the same messages start appearing on slides throughout the document.
Over time, the company positioning can change a bit. Rather than starting with a clean sheet, the old presentation gets adjusted. The result: a bit of the old, a bit of the new.
Do spring cleaning now and then.